Change VAT calculation Basis

Hi,

I need to calculate VAT amount not on the full balance of the document. Related to AR Invoice, once the invoice created advance will be deducted from the invoice. I need to calculate VAT on the balance amount after applying the prepayment to the invoice. 

Example.

The Progress Bill (AR Invoice) - 1000 $

Prepayment deducted              -200$

Balance Receivable                  -800$

 In the system it does calculate VAT on 1000$. I need to calculate VAT on 800$. How is this possible in the system .

Thank you,

Dulanjana