sales order

Hi,my user has made the sales order and completed it till sales update.cost of order was 500.  Later he got to know that  there was some mistake. so in the same sales order he put the negative line and again taken the order till sales update.This way the same 500 was credited back to the customer..
the cycle was completed.

My question is if there would have been the difference of exchange rate since both the sales update was done on different dates.  where the difference of that money would have gone. how to deal this problemin jd?

Thanks and regards