prepayment invoice and standard invoice for withholding tax

hi dears

i have created prepayment, and system deducted withholding tax on this. now i want to create a standard invoice, however i want that after application of prepayment, system should calculate tax on additional amount

for example,

prepayment 5million and tax deducted

standard invoice is 7 million

when i am entering standard invoice, i want to apply WHT only on 2 million.

thanks

Ali