The setting for using custom exchange rates can be found under Settings → Accounting → Calculation → Allow Custom Exchange Rates.
For converting foreign currency amounts on receipts, Spesenfuchs automatically uses exchange rates provided by openexchangerates.org. The exchange rate is determined based on the receipt date and used to calculate the amount in euros.
If the Allow Custom Exchange Rates option is enabled in the settings, users can replace the automatically determined exchange rate for a receipt with their own exchange rate. The euro amount is then recalculated based on the custom rate.
The entered exchange rate must be greater than 0. If a custom exchange rate differs by more than 30% from the automatically determined system rate, Spesenfuchs displays a warning. However, the user can still save and use the entered rate.
For more information about entering receipts in foreign currencies, see Enter Receipt.