Solution : https://service.sap.com/sap/support/notes/449666 (SAP Service marketplace login required)
Key words : 
additional key words apo, prerequisites solution implement, source code correction, -party order processing, apo backorder processing, sales order item, apo system, backorder processing, sales order, purchase order
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting |