When you enter a bill in Accounting, choose its currency and enter the lines as the supplier wrote them. Also enter what the bill was worth in the books' currency. The bill keeps its balance in its own currency.
When you pay it, enter the amount in the bill's currency and what left your account. Any difference goes to Exchange gains and losses, and the last payment clears the bill exactly. Aged payables, Search and each supplier's contact page show each bill in its own currency.