When a debt is incurred in one currency to be repaid in another, what date should determine the exchange rate?

To clarify further: Exchange rate at date of original transaction, or exchange rate at date of repayment? Other?|||it should be at the time of payment. unless otherwise specified in the contract.|||Original transaction date.|||Exchange rate at the date of payment. I'd recommend a forward or futures contract to limit the exchange rate risk if the payment is to be made very far into the future and the amount is large.|||I'd lock in the exact amount at the time the contract is executed, i.e., the original transaction. Then the payee knows what he is owed and can invest / hedge accordingly. As long as the date is agreed in the contract, either date will work (which is why this is even a question, right?).