If you raise a Purchase Requisition with an incorrect quantity and generate a PO from it, theres no way to correct the PO. If you try to amend the Purchase Order and reduce the quantity below the original amount you get a message advising that the quantity cant be reduced below the original Purchase Requisition quantity. That means you have to receive and invoice the goods, then raise a return to fix a simple mistake.
If you select to reduce the quantity on the order you will be presented with 2 options:
Yes cancel the requisition Line
No - only reduce quantity on the order. The requisition can be re-ordered.