Sub-Ledger is a detailed accounting record that holds every individual transaction or balance behind a single account in the general ledger, showing which customer, invoice or wallet makes up a total such as customer funds owed, and the two must always agree. Transactions post to it at full detail, and it sends summarised journals to a control account, in real time or in batches; any difference must be investigated before the books close. At payment and e-money firms, the customer balance sub-ledger records what is owed to each client. Under CASS 15, in force since 7 May 2026, daily safeguarding reconciliations, monthly returns and resolution packs all depend on it, and buyers test its daily reconciliation history.
How does a sub-ledger work
Transactions are posted first to the sub-ledger at full detail: customer ID, amount, currency, timestamp, reference. The sub-ledger then posts a summarised journal to a control account in the general ledger, either in real time or in a periodic batch. If the sum of all sub-ledger balances does not equal the control account, something was posted to one and not the other, and the difference must be investigated before the books close.
Traditional businesses run sub-ledgers for receivables, payables, fixed assets and inventory. Payment and e-money firms add the most important one: the customer balance sub-ledger, which records what the firm owes each client.
Sub-ledger vs general ledger
The general ledger is built for financial statements, so it is organised by account and summarised. A sub-ledger is built for operations, so it is organised by counterparty and can hold millions of rows. A payment ledger at a fintech typically acts as the sub-ledger: it records every wallet movement in double entry and feeds daily totals per currency into the general ledger.
Why sub-ledgers matter for safeguarding
For a UK payment or e-money institution, the customer sub-ledger is the source of the safeguarding requirement. Under the FCA's CASS 15 regime, in force since 7 May 2026, firms must reconcile safeguarded funds every business day, file a monthly safeguarding return and keep a resolution pack so client money can be returned quickly if the firm fails. All three depend on a sub-ledger that shows, per customer and currency, what is owed.
A worked example: an EMI's sub-ledger shows 18,400 customer wallets totalling €6,215,400 at close of business. Its safeguarding account statement shows €6,190,000. The €25,400 shortfall must be explained, typically by card settlements in transit or an unposted refund, and topped up from the firm's own funds if it is a genuine deficit. Under the old monthly-reconciliation habits of many small EMIs, a gap like this could sit unnoticed for weeks; daily reconciliation against the sub-ledger surfaces it the next business morning, while the transactions behind it are still easy to trace.
Sub-ledgers in due diligence
When a licensed fintech is sold, the buyer's accountants test whether the customer sub-ledger reconciles to the general ledger and to bank statements for every day of the review period. A sub-ledger maintained in spreadsheets, or one that only reconciles monthly, signals safeguarding risk, a likely remediation cost and, under CASS 15, a possible conversation with the regulator before the change-in-control approval.