Direct Shipment is used in situations where the supplier ships goods directly to the customer. 
In this process, the customer sales order link is basically locked to the purchase order (SPO or SOA) and is considered a special 1-1 order. 

It is not possible to relink goods reception items (SSA-items) to other customer orders items when the goods reception item (SSA-item) is created in this process and flagged as 'direct shipment'.

If this is required, one must use the workaround of cancelling the customer order (COA), and write off the goods reception (SSA) using stock correction and add the quantity manually to stock on a new goods reception (SSA).