I have always used credit note to recognise withholding tax on customer transactions. I fill the credit note and change the product/service to a withholding tax receivable GL I created. That way, it reduces the customer's balance (without altering my revenue) and increase my withholding tax receivable account (asset). Now, this is not possible again while filling the credit note. The product/service cannot be changed again and thats a bit challenging to recognise withholding tax.