Ability to amend quantities on back to back orders Merged

When generating a Purchase Order from a Sales Order, it is not possible to amend the quantities. If the stock is short delivered, there is then no way to Complete the Sales Order and Purchase Order. As an example, a quantity of 40 is entered on a Sales Order and therefore the Purchase Order is generated with the same quantity.Only 36 are received, so the goods are booked in on the PO side. The additional 4 is never going to be received. The quantity cannot be reduced on the SO and also cannot be reduced on the PO. Even before booking the goods in, you can reduce the quantity on the Sales Order but still cannot amend the Purchase Order quantity. It is not possible to write off the Purchase Order as the order is linked. The system should allow quantities to be amended so that both the SO and PO are not sat in the system as outstanding.