Your fintech holds USD reserves at a sponsor bank ($1,000) for cross-border payouts. On your NGN books, that USD was booked at the rate the day it landed: 1500 NGN/USD = ₦1,500,000. A month passes. NGN devalues; rate is now 1600. The SAME $1,000 is now worth ₦1,600,000 on your books, BUT your ledger still says ₦1,500,000. The gap (₦100,000) is an unrealised FX gain that has to be posted, or your balance sheet lies. This is FX revaluation: at period close, every foreign-currency asset and liability is re-marked at the spot rate; the gain or loss flows to a P&L account.