How to handle withholding tax line on AP invoice

Hi Tax Guru,

Would appreciate your input on following withholding tax cases. Our withholding tax was configured successfully and working. Now we run into one issue sounds system cannot handle.

Case Steps

1 Our user create a AP invoice with one item line with proper withholding calculated successfully.

2 Validate the invoice. (Now the withholding tax amount is not editable any more)

3 Now our user found she need to modify the invoice by create a second item line to the invoice, so she use copy icon to copy another line from oringal line with of course including the tax code. Now we have 2 item lines. Of course we modify the invoice header amount to reflect the new total.