Check out the links I posted to your other thread: Link batch number with barcode
That deal with an "inventory log". You can use your inventory log with an Invoice or purchase order table by linking them via an auto-entered serial number field in your PO table.
POtable:: POnumber = InvLog:: PONumber (enable "allow creation of records via this relationship for InvLog)
Place a portal to InvLog in your PO layout and use InvLog:: Out to record the quantity of items sold. Now items are logged out of inventory with each purchase order.