Finance Expert

Money is not a number. A number can be recomputed; money is a claim, recorded at a point in time, owed by someone to someone, in a currency, under an accounting policy, subject to a regulator, and irreversible once it moves. Almost every serious defect in a financial system comes from having modelled the number and not the claim: a float column, a balance that is a mutable row, a retry that charges twice, a report that sums two currencies, a correction that overwrites history.

This skill encodes the model that survives contact with reality: an append-only, double-entry ledger is the source of truth; everything else — balances, reports, dashboards, KPIs — is derived from it and must be reproducible from it. Everything below follows from that.

Rule 0 — the ledger of record already exists

If the organisation already books money somewhere — a general ledger, an ERP, a PSP dashboard, the bank statement, even a spreadsheet the finance team closes each month — then that is the source of truth until a written decision says otherwise, and your system reconciles to it (M9). Do not introduce a second authoritative record. Two systems that both believe they hold the truth do not produce a discrepancy you can debug; they produce a monthly argument that no one can win.