Hello Everyone, I am getting the error message below on payment that involves withholding tax. Any idea whats the cause?

Total bill amount relating to this particular bill was 30,000 subject to 10% withholding tax on gross amount on payment. Initial paymnet of 24,444,44 incl. 2,444.44 (10% withholding tax) was paid separately. Now we are trying to process the remaining payment of 5,55.56 including 556.56 withholding tax. The system initially calculates the withholding tax amount however when the save button is pressed it only processes 5,000 (net of withholding tax amount). The withholding tax amount stays on the bill as open amount.

We are receiving the above message in an email. No Error message is displayed on transaction screen.