Solution : https://service.sap.com/sap/support/notes/512646 (SAP Service marketplace login required)
Key words : 
symptom temporary quantity assignments, oltp sales document, rejected solution implement, implement note 517897, backorder processing, terms transactions, /sapapo/bop, /sapapo/bopi, /sapapo/bop_updatesent
Related Notes : 
       
| 517897 | Long runtimes when updating sales orders | 
| 509865 | Backorder processing: Temporary quantity assignments | 
| 500637 | Backorder processing: temporary quantity assignments | 
| 375193 | Backorder processing: Composite SAP note on posting |