WHT tax applicabale at the time of prepayment applied

 Hi,

I create a prepayment invoice Rs 100 and thier payment

Now i create standard invoice with the WHT 6% code and apply the prepayment.It should deduct 6% WHT tax amount on applied prepayment invoice amount.Mean Rs 94 should be applied after deduction of Rs 6.

But it is applying complete Rs 100 to standard invoice and not deduciton Rs 6(WHT amount)