not able to apply prepayent tax to standard invoice

Hi all,

we want to apply prepayment to our standard invoice. we understand that we can apply only to the extent of 'ITEM' line type of prepayment to standard invoice . But we have tax portion in our prepayments to be applied to standard invoice. so, I am not able to adjust TAX portion of prepayment to standard invoice . Thais is stopping our payment as well. ( because we are not able to derive net amount to be paid) . so, please help us to solve this issue. we have many prepayments( which will run for years -- with big numbers)