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The account and product model, the double-entry ledger, interest and fee accrual, and end-of-day processing.
3 questions in Banking & Financial Services.
Ledger balance is posted history; available balance is the money the customer can use after holds, uncleared items, earmarks and overdraft limits. Define it once so every banking channel shows the same number.
Store money as immutable double-entry rows where every movement writes at least two balanced entries, derive balances by summing those entries, correct mistakes with compensating entries instead of updates, and keep each currency in its own denominated account so nothing is ever summed across units.
Prove the new platform's opening position equals the old platform's closing position, account by account and in control totals, before you accept a single instruction on it. Balances are the tractable half - standing instructions, accrued interest and payments in flight across the boundary are what break.