You don't say what version of Sage you're using. Some have have modules that will calculate the value of the invoices using a exchange rate you have already pre set. When payment is received or made, any differences are posted to an account in the nominal ledger (around 7900ish) called exchange rate variance.
For clients that don't have the foreign exchange rate module, I use a foreign exchange website called Oanda to calculate the invoices in Sterling & again post any exchange rate differences to the exchange rate variance nominal account.
I hope this helps!