TAX Deducted @ Sources

Dear All

As SAP provides the standard withholding tax solution on incomming payments. but my problem is that at the time of incomming payment i can't post the withholding tax amount becuase my customer don't provide me the payment detail (Remittance advice). please suggest me what can be the process and how i can post the withholding tax deducted by customers.

my country is pakistan and currently, no country specific solution is available for withholding tax.

Thanx