Hi Mohammed,
Have you checked the exchange rate on the lines? I have noticed that if you have multiple lines with different exchange rates it can cause an issue as exchange rates often work to more that 2 decimal places. with the double precision versus 3 decimal places it can cause slight differences in the overall voucher balance...
If you personalise the screen and move the exchange rate to the main form and then export to excel (CTRL +T) you can manually work out the balances and see if there is a difference.
Im not sure if this will help but its worth a check...
Kind Regards,
ObiOne