At my firm we often receive bills in foreign currencies from vendors. When we go to pay the bills in QuickBooks the exchange rate has frequently changed from what it was when we received the bill. So we pay the bill and now it either looks like we haven't paid the bill in full or it looks like we've overpaid. 

This is further complicated by the fact that we include 'billable' bills on the invoice to our clients. Our process looks like this: We receive the bill and then invoice our clients first for the amount of the bill and then after the client pays us we pay the vendor. In that time the exchange rate has changed several times and the amount we are paying is no longer the same as the amount of the bill on the invoice. I'm considering changing the amount of the bill to reflect the rate on the date we pay it???

If you can make sense of this somewhat jumbled question I'd really appreciate any advice on resolving this issue.