Accounting Systems and Source Documents

Learning objectives

By the end of this chapter, you should be able to:

• Explain how source documents provide evidence for transactions and support reliable financial records.
• Distinguish between manual, spreadsheet-based, and computerised accounting systems, and evaluate their advantages and risks.
• Apply practical controls that improve accuracy, reduce error, and create a clear audit trail.
• Detect common document and posting errors before they affect ledger balances and the trial balance.
• Trace a transaction from source document through prime entry records and ledgers into the financial statements.

Overview & key concepts

Accounting records are only as reliable as the evidence and processes behind them. Source documents provide that evidence: they show what happened, when it happened, with whom, and for how much. An accounting system then converts those documents into double-entry postings through prime entry records and ledgers, producing totals for the financial statements.

This chapter explains:

• what common source documents are used for,
• how transactions flow through books of prime entry, subsidiary ledgers, control accounts, and the general ledger, and
• how basic controls prevent errors such as duplication, omission, or incorrect classification.

Source documents

Source documents are the original records that support a transaction. They are used to justify entries, confirm amounts, and provide dates and references for posting.

Typical examples include:

• Sales invoice: evidence of a sale and the amount owed by a customer (often a credit sale).
• Purchase order (PO): an authorised instruction to a supplier to provide goods or services. A PO is not, by itself, an accounting entry, but it supports authorisation and later matching.
• Goods received note (GRN): confirmation that goods have been received. It supports completeness (recording what has arrived) and matching (checking supplier invoices).
• Supplier invoice: evidence of a purchase and the amount owed to a supplier.
• Credit note: reduces a previously invoiced amount (e.g. returns, allowances, pricing corrections).
• Remittance advice: explains which supplier invoices are being paid.
• Bank statement: external evidence of bank movements and balances, mainly used for bank reconciliation rather than as a substitute for the cash book.

Link to the accounting equation

Every posting affects the accounting equation:

Assets = Liabilities + Equity

Income and expenses affect equity through retained profits. For example:

• A credit sale increasesreceivables (asset)and increasesincome (profit → equity).
• A supplier invoice increasespayables (liability)and increasesinventory (asset)(or expense, depending on what was purchased).
• A credit note issued to a customer reducesreceivables (asset)and reducesincome(or records a sales returns contra-income).

Transaction cycles

A transaction cycle is a repeatable process that generates transactions and documentation. Common cycles include:

• Sales cycle: customer order → dispatch → sales invoice → collection → receipt evidence (bank entry/remittance).
• Purchases cycle: requisition → purchase order → receipt of goods (GRN) → supplier invoice → payment.
• Payroll cycle: time records/authorisation → payroll calculation → payment and statutory deductions.

Understanding cycles helps you:

• know which document should exist at each stage,
• identify missing steps (e.g. goods received but no invoice recorded), and
• apply appropriate controls (e.g. matching and authorisation).

Audit trail

An audit trail is the ability to trace a transaction:

• from source documents to the financial statements(to confirm that recorded amounts are supported), and
• from the financial statements back to source documents(to confirm that amounts can be evidenced).

A strong audit trail is supported by:

• sequentially numbered documents (and checks for missing numbers),
• clear cross-references (invoice number, customer/supplier reference), and
• retention of documents in an organised way (paper or electronic).

Prime entry books and ledgers

Books of prime entry (day books / journals)

Prime entry records list transactions in chronological order before posting to ledgers. Common examples:

• Sales day book: credit sales invoices issued.
• Sales returns day book: credit notes issued to customers.
• Purchases day book: credit purchases (supplier invoices received).
• Purchases returns day book: supplier credit notes received.
• Cash book: bank and cash receipts and payments (often treated as both a prime entry book and a ledger).

Some systems post individual invoices directly to ledgers without using day books as visible “books”. Day books remain a useful teaching model for understanding posting flows, totals, and audit trail structure.

Day books typically record totals for posting to the general ledger, while individual customer and supplier balances are posted to subsidiary ledgers.

Subsidiary ledgers and control accounts

• Thereceivables ledgerholds individual customer accounts.
• Thepayables ledgerholds individual supplier accounts.
• Thereceivables control accountandpayables control accountare general ledger summary accounts that should agree to the totals of the subsidiary ledgers after reconciliation.

Manual vs spreadsheet vs computerised systems

Manual systems

Strengths

• simple to understand and transparent,
• easy to see document flow physically.

Risks

• arithmetic errors, poor legibility, and posting mistakes,
• duplication and omission if document control is weak,
• time-consuming for high transaction volumes.

Spreadsheet-based systems

Strengths

• flexible and low cost,
• useful for small volumes or analysis.

Risks

• formula errors (wrong ranges, broken links),
• version control issues (multiple conflicting files),
• weak access controls unless well managed.

Computerised systems

Strengths

• fast processing, automatic postings, and built-in validation (e.g. balanced journals),
• easier reporting and audit trail features (if configured well),
• can enforce user permissions and workflow.

Risks

• incorrect setup (codes, tax treatment, posting rules) produces systematic errors,
• over-reliance on defaults,
• access/security weaknesses if permissions and approvals are not controlled.

Core theory and frameworks

Recognition and measurement from source documents

Effective processing involves two steps:

1. Recognition (what is it?)
   • Identify the document type (invoice, credit note, GRN, PO).
   • Confirm key fields: date, parties, reference numbers, description, quantities, prices, totals, and approvals.
2. Measurement (what is the accounting impact?)
   • Determine the accounts affected and the amounts to record.
   • Decide whether the transaction is cash or credit, and whether it affects assets, liabilities, income, or expenses.

Double-entry logic and debit/credit rules

Double entry means every transaction has at least one debit and one credit, and total debits equal total credits.

A practical way to avoid confusion is to link debits/credits to the accounting equation:

• Assets: increase withdebits, decrease withcredits.
• Liabilities: increase withcredits, decrease withdebits.
• Equity: increases withcredits, decreases withdebits.
• Income: increases withcredits.
• Expenses: increase withdebits.

Cash vs credit transactions

• Acash sale:
  • Dr Bank/Cash
  • Cr Revenue
• Acredit sale:
  • Dr Receivables
  • Cr Revenue
• When the customer later pays:
  • Dr Bank/Cash
  • Cr Receivables

Do not credit bank for a credit sale—bank only moves when cash is actually received.

Classification tests that commonly appear in exams

Capital vs revenue

• Capital expenditure: creates or improves a long-term asset (record as an asset, then depreciate/amortise).
• Revenue expenditure: day-to-day operating cost (record as an expense when incurred).

Inventory cost vs period costs

Treat a cost as part of inventory when it is incurred to acquire or prepare goods so they are ready to be sold. If the cost is necessary to obtain the goods and make them saleable, it is usually included in inventory until the goods are sold.

Costs that relate to running the business overall—for example routine selling and marketing, head office administration, and most storage that happens after the goods are ready for sale—are normally recognised as expenses when incurred.

Example: supplier price, import duties, and inbound delivery on purchase are typically inventory-related; sales staff salaries and advertising are period costs.

Inventory and cost of sales

• Inventory increases when goods are purchased and decreases when goods are sold or otherwise issued.
• Cost of sales is recognised when the related revenue is recognised, based on the cost of the goods sold.

Common timing areas

Deferred income (unearned revenue)

If a customer pays in advance for goods or services not yet provided, the business has an obligation:

• Dr Bank
• Cr Deferred income (liability)

Revenue is recognised when the goods/services are delivered:

• Dr Deferred income
• Cr Revenue

Notes payable and interest

A note payable is a borrowing or formal debt:

• On issue: Dr Bank / Cr Notes payable
• Interest accrues over time, even if unpaid:
  • Dr Interest expense
  • Cr Interest payable (liability)

Allowance for doubtful debts

Receivables are shown at an amount expected to be collected:

• Create or increase allowance:
  • Dr Bad debt expense (impairment loss)
  • Cr Allowance for doubtful debts (contra receivable)
• Write off a specific irrecoverable balance:
  • Dr Allowance for doubtful debts
  • Cr Receivables

Worked example

Narrative scenario

ABC Retailers operates a small retail business.

Assumptions (for this question):

• Opening receivables and payables arenil.
• Amounts areexclusive of VAT/sales taxunless stated.
• The customer credit note on 18 January relates toInvoice #001.

The following events occurred in January:

1. Goods are dispatched to a customer on5 Januaryand asales invoiceis issued for£1,000(Invoice #001).
2. Apurchase orderis approved for goods costing£500from Supplier X on10 January(credit purchase).
3. Goods are received from Supplier X on12 Januaryand agoods received noteis prepared.
4. Supplier X’sinvoicearrives on15 Januaryfor£500.
5. The customer returns goods on18 Januaryand acredit noteis issued for£100againstInvoice #001.
6. Payroll for January is processed and paid on25 January, total£2,000.
7. The customer pays by bank transfer on28 Januaryto settleInvoice #001 net of the credit note(payment£900).
8. Aduplicate copyof Supplier X’s invoice is mistakenly entered again on30 January.
9. Abank statementis received on31 Januaryshowing a bank balance of£10,000.
10. A secondsales invoiceis issued on31 Januaryfor£1,200(Invoice #002).

Required

1. Compute the total sales revenue for January.
2. Prepare the sales day book and purchases day book for January.
3. Reconcile the receivables ledger with the receivables control account at 31 January.
4. Identify and correct the duplicate invoice entry.
5. Describe the impact of these transactions on the financial statements.

Solution

1) Total sales revenue for January

Invoices issued (gross sales):

• Invoice #001: £1,000
• Invoice #002: £1,200
• Gross sales invoiced = £2,200

Customer credit note issued (sales returns/allowances):

• Credit note against Invoice #001: £100

Net sales for January:
Net sales = £2,200 − £100 = £2,100

Answer:

• Gross sales invoiced:£2,200
• Net sales after returns/allowances:£2,100

2) Sales day book and purchases day book for January

Sales day book (credit sales invoices only)

Posting summary (from sales day book total):

• Dr Receivables control account £2,200
• Cr Revenue (sales) £2,200

Sales returns day book (credit notes to customers)

Posting summary (from sales returns day book total):

• Dr Sales returns/allowances (contra revenue) £100
• Cr Receivables control account £100

Purchases day book (supplier invoices on credit)

Posting summary (as entered, before correction):

• Dr Inventory (or Purchases) £1,000
• Cr Payables control account £1,000

3) Reconcile receivables ledger with receivables control account at 31 January

Because opening receivables are nil, reconciliation is based on January movements.

Receivables movements (January):

• Credit sales invoices issued: +£2,200
• Less credit note issued: −£100
• Less customer receipt (net settlement of Invoice #001): −£900

Closing receivables:
£2,200 − £100 − £900 = £1,200

Receivables control account (general ledger summary)

• Debit:Sales invoices £2,200
• Credit:Sales returns/allowances £100
• Credit:Bank receipts from customers £900
• Balance c/d (debit):£1,200

This should agree to the receivables ledger total. Here the customer position is:

• Invoice #001 £1,000 less credit note £100 = £900, paid on 28 January →nil outstanding
• Invoice #002 £1,200 issued 31 January →£1,200 outstanding

Reconciled closing receivables: £1,200

Common reconciling items (what usually causes differences):

• Cash receipt posted to the wrong customer account in the receivables ledger
• Credit note issued but not posted (or posted to the wrong customer)
• Invoice included in one record but not the other due to timing of posting
• Incorrect contra entries (e.g. set-off) posted in the ledger but not reflected in the control account total

4) Identify and correct the duplicate supplier invoice entry

Error: Supplier X’s invoice for £500 was entered twice on credit.

If the incorrect duplicate posting was:

• Dr Inventory/Purchases £500
• Cr Payables £500

Then reverse the duplicate:

• Dr Payables £500
• Cr Inventory/Purchases £500

After correction:

• Purchases recorded for Supplier X in January:£500
• Payables owed to Supplier X at 31 January (assuming unpaid):£500

5) Impact of these transactions on the financial statements

Statement of profit or loss (January)

• Revenue (gross):£2,200
• Less sales returns/allowances:£100
• Net sales:£2,100
• Payroll expense:£2,000

Purchases are recorded from supplier invoices, but cost of sales depends on what goods were actually sold and the cost assigned to those goods. This scenario does not provide enough information to calculate cost of sales, so it should not be guessed.

Statement of financial position (at 31 January)

• Receivables:£1,200 (Invoice #002 outstanding)
• Payables:£500 (Supplier X outstanding, after removing the duplicate)
• Bank:increases by the customer receipt of £900 and decreases by payroll of £2,000, subject to any other bank movements not described.

Inventory and customer returns (what can and cannot be concluded)

The purchases invoice supports that goods costing £500 were acquired on credit. However, the closing inventory figure cannot be determined from the information given because it depends on additional assumptions, including:

• whether the goods sold on 5 January included items from the £500 purchase (timing and costing assumption),
• whether the customer return on 18 January is returned to inventory in saleable condition, and
• whether the system records inventory continuously (perpetual) or only at period-end (periodic).

What you can say safely:

• The supplier invoice affects inventory (or purchases) and payables at £500.
• The customer credit note affects revenue (via sales returns/allowances) and receivables at £100.
• Any inventory effect from the customer return depends on the inventory system and whether the returned goods are restocked.

Bank statement balance

The bank statement balance of £10,000 is external evidence and is mainly used for bank reconciliation. The statement figure should not be posted as a journal entry simply to “make bank equal £10,000”; differences are explained through reconciliation.

Common pitfalls and misunderstandings

• Treating a purchase order as an accounting entry: a PO supports authorisation but does not, by itself, create an asset or liability.
• Mixing up invoices and credit notes: a credit note reverses part or all of an earlier invoice; posting it the wrong way round overstates revenue and receivables.
• Confusing cash and credit: issuing an invoice creates a receivable; cash only appears when payment is received.
• Recording bank statement balances as journal entries: statements support reconciliation; they are not a substitute for posting receipts/payments from the cash book evidence.
• Omitting goods received when invoice is delayed: where procedures require it, goods received may need to be accrued so liabilities and inventory are complete.
• Duplicate invoice posting: a frequent cause of overstated inventory/expenses and payables; prevent with matching and duplicate checks.
• Control account not reconciled to subsidiary ledger: differences often arise from posting to the wrong customer/supplier, missing postings, or timing errors.
• Payroll entries oversimplified: payroll often includes deductions that create liabilities; do not assume payroll is only “wages expense and bank”.

Summary and further reading

Source documents provide the evidence that transactions occurred and support accurate posting. Accounting systems—manual, spreadsheet-based, or computerised—turn those documents into double-entry records through prime entry books and ledgers. A strong audit trail depends on controlled documentation (numbering, authorisation, retention) and routine reconciliations (control accounts, bank).

This topic supports broader areas such as ledger reconciliation, error correction, and preparing reliable financial statements.

For further reading, use introductory financial accounting resources focusing on double entry, ledger systems, internal controls, and reconciliation techniques.

FAQ

What is the significance of source documents in accounting?

They provide evidence for what happened and support the amount, date, and parties involved. They also make it possible to trace transactions through the records and confirm that ledger balances are supported by underlying documentation.

How do manual, spreadsheet-based, and computerised systems differ?

Manual systems rely on physical records and hand-posting, which increases the risk of arithmetic and posting errors. Spreadsheets are flexible but vulnerable to formula mistakes and version control issues. Computerised systems process faster and can enforce validation rules, but incorrect setup or weak access controls can cause systematic errors.

What key controls improve accuracy and strengthen the audit trail?

Common controls include document authorisation, sequential numbering, restricted access, independent checks, and regular reconciliations. For purchases, matching the PO, GRN, and supplier invoice helps ensure only valid and complete purchases are recorded.

Why is an audit trail important?

It allows a transaction to be followed from evidence (source documents) through postings into the financial statements and back again. This supports verification, reduces error risk, and improves confidence in reported balances.

How can duplicate invoice entries be prevented?

Use duplicate checks (invoice number/supplier), stamp or lock documents once entered, and apply matching procedures so invoices cannot be posted without supporting authorisation and receipt evidence. Review exception reports and reconcile supplier statements where available.

Summary (Recap)

This chapter explains how source documents underpin reliable accounting records and how transactions flow through prime entry books, ledgers, and control accounts to reach the financial statements. It compares manual, spreadsheet, and computerised systems, highlighting where errors and fraud risk commonly arise. It also sets out practical controls—authorisation, numbering, matching, and reconciliations—that protect completeness and accuracy. The worked example demonstrates credit sales, sales returns, customer receipts, credit purchases, and the correction of a duplicate supplier invoice entry, linking document evidence to double-entry postings and ledger reconciliations.

Glossary

Source document
Original evidence supporting a transaction and its details (date, parties, amount, reference). Examples include invoices, credit notes, GRNs, and bank evidence.

Transaction cycle
A repeatable process (e.g. sales, purchases, payroll) that creates transactions and documentation from initiation through to recording and settlement.

Audit trail
A clear, traceable link between source documents, accounting records, and reported figures, enabling verification in both directions.

Prime entry book
A chronological listing where transactions are first recorded (e.g. sales day book, sales returns day book, purchases day book, cash book) before posting to ledgers.

Sales invoice
Document issued to a customer requesting payment for goods or services supplied; commonly used to support revenue recognition and receivable recording.

Credit note
Document that reduces a previously invoiced amount, typically for returns, allowances, or billing corrections.

Purchase order (PO)
An instruction authorising a purchase from a supplier; supports approval and matching but is not normally posted as an accounting entry by itself.

Goods received note (GRN)
Evidence that goods have been received; used to check quantities and support matching to supplier invoices.

Remittance advice
Document accompanying or following payment that identifies which supplier invoices are being settled.

Control account
General ledger summary account representing the total of a subsidiary ledger (e.g. receivables control account totals all customer balances).

Subsidiary ledger
Detailed ledger containing individual customer or supplier accounts that support the related control account total.

Batch processing
Grouping transactions for processing and checking as a unit, helping completeness checks and control over document handling.

Payroll template (high-level posting idea)
A common posting structure (amounts depend on the data provided):

• Dr Payroll expense (gross pay)
• Cr Deductions payable (tax/social security/other withholdings)
• Cr Bank (net pay)