Solution : https://service.sap.com/sap/support/notes/367039 (SAP Service marketplace login required)
Key words : 
additional key words /sapapo/sales 600, attached source code corrections, atp check occurs, prerequisites solution implement, sales order, /sapapo/sales600, implement note 371628, apo system, foreign system, system generates
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting | 
| 371628 | Manual backorder processing in APO for R/3 orders |