Once a supplier shipment is reserved, it needs to be picked from the specific locations. This is done on Supplier Shipment page with Report Picking. You cannot pick with differences with the supplier shipments. In rental returns, if you have reserved a partial quantity out of what has been originally planned to return, the planned quantity to be returned will be reduced to match the picking quantity. If you have incorrectly reported the supplier shipment as picked, then you will have to cancel the whole shipment and start the process from the beginning.