To be able to raise sales invoice for back to back direct delivery order, before stock is received

As standard if a back to back order is raised, with PO generated from SO, it requires confirmation of direct delivery of stock for the PO, before the invoice can be raised against the SO. Customer would like the option to invoice the SO early, before stock has been recorded against the PO. Requirement to provide customer with invoice and post this to their ledgers straight away, as payment is being received up front for some orders, before the stock can be recorded on the PO.