Updating Assignment field with PO number in FI payment documents

Hi,

I have situation on hand where Sort Key in vendor master was not updated with value 010 (i.e. copy PO number in FI documents as reference).

Payments were cleared for a number of vendors where link between payment, Invoice (from MM) and PO is broken.

Now I need to fix these FI payment documents by updating the assignment field in them with PO number so that FBL1N report shows the PO number under assignment column.

Is there any standard program available in SAP that updates assignment field with PO number in FI payment document?

Regards,

Ali