Business case :

• Purchase Order Has Been Created for Vendor X , Product Line for material Z=10 Quantity

• We make a shipments for 10 Quantity .

• then we make return of the same shipments for 10 Pieces and we update the check box for Update P.O.

• the result will happen that Purchase order Receiving Quantity is 0  that totally correct .

• now i want to receive some Quantity  on the same purchase order i can't Odoo does not allow me to enter any quantity.

• in version 10 there was workaround that just duplicated and enter the shipment