Unable to update Withholding tax rate code on invoice before partially paid

Dear Experts,

Your input in required in this requirement.

PROBLEM

Unable to update Withholding tax rate code on invoice before partially paid.

User Requirement:

System has allow user to update tax rate on invoice at any stage before make the payment.

SYSTEM CURRENT BEHAVIOR

For Example

1. User has create an invoice with the total amount of 1000 and define W.tax rate code 10% on it.
2. Validated & accounted invoice
3. Partially paid with the amount of 600 and remaining amount 400 will paid later.
4. At the stage user want to update tax code 12% and paid the remaining amount but unfortunate system freeze the invoice and not allow any adjustment on it.