Once you have partially dispatched an sale and haven't already raised a full invoice, the stock sale will display the Create Backorder function.
Clicking the Create Backorder button will create another draft stock sale and transfer over all all the products not yet dispatched. The new stock sale will have the original sale's reference with the /x suffix, where x is the number of times the sale has been backordered. Approve backorder and invoice and dispatch using the normal stock sale workflows independently from the original order.
The original stock sale will be marked as fully dispatched and will need to completed by raising an invoice for the partially dispatched products. Note: The original order will still retain the original product line items for reporting, but with the value set to $0.
Add a payment to the approved invoice if required.