how to enter the tax element when matching prepayment entries to invoice in 11i

Can anyone explain how to enter the tax element when matching prepayment entries to invoice. I have entered the prepayment invoice and added the tax to the net amount. Now when entering the invoice andsplitting out the tax element and applying the prepayment amount I cannot validate. The error message says the distributions do not match invoice amount. It must be around the tax but not sure how to entered it correctly.

Regards

Edwina