Exchange rate difference

Hello,

we have faced a problem in one project. Due to country financial legislation, every document (issued, or got) in different exchange rate from the local, has to be reevaluated every month. Every end of the month - exchange rate difference has to be calculated and put to some proper GL account and customer debt account. This reevaluation happens every month, and when the final payment of CO invoice or some AP document comes, the exchange rate is calculated only from last reevaluated amount...
M3 however supports only the exchange rate difference between received (issued) doc. and it's payment...
Any suggestions, how this could be solved with minimum effort ?...

thanks for all comments