Solution : https://service.sap.com/sap/support/notes/335233 (SAP Service marketplace login required)
Key words : 
object exists -> 'c' solution implement, external key  object, transfer sales orders, additional key words, method 'c', exist -> 'n', prerequisites, is_sl_head
Related Notes : 
       
| 375193 | Backorder processing: Composite SAP note on posting |