Back-to-back Ordering - Direct Deliveries - Override Unit Quantity

When generating a Purchase Order from a Sales Order with 'Direct Delivery' fulfilment method, once generated neither the Sales Order or Purchase Order unit quantity can be changed. For one of our Clients there are instances when the precise unit quantity is not known until the goods have been weighed upon delivery, however as the unit quantity is locked for editing, the workaround is not ideal. When the quantity is found to be under the quantity on the SO/PO, there are steps to write off the stock to complete the order (Ask Sage article 29497 relates), and when the quantity is found to be over the quantity on the SO/PO, then an additional item line needs to be added to the SO and another PO generated. Can the unit Quantity field be unlocked on the generated PO, and changes made to the PO to automatically update the linked SO?