How to specify whether the system applies taxes to the retained amount during the voucher match proc

Hello ,

I am currently working on a client site who requires that the VAT applies on the total amount as a against the subtotal in a complex PO matching. How to specify whether the system applies taxes to the retained amount during the voucher match process and not when releasing retained amounts.

Example:

PO Amount: $400

Retainage Amount: $100

Vat : 10%

Expected result

Vat Amount: $40

Actual result

VAT Amount: $30

How can i ensure that VAT rate applies only to the total sum and no VAT on the retained amount when i want to release the funds

Please assist.