Settle-up transfers are stored as fixed amounts, but a foreign-currency expense was re-converted with live rates on every settlement calc. When the rate drifted, the fixed transfer no longer cancelled the re-valued expense and a few-cent residual re-opened the settled position. Foreign-currency expenses now freeze the live rate at entry time into the existing budget_items.exchange_rate column, and the settlement converts with that frozen rate when working in the trip currency. Legacy rows (exchange_rate = 1) keep using live rates, so historical data is unchanged until re-edited; rate fetch failures fall back to live rates.
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