Solution : https://service.sap.com/sap/support/notes/95760 (SAP Service marketplace login required)
Key words : 
additional key words subsequent settlement, underlying purchase order item, vendor business volumes, volume-based rebate, purchase order item, vendor business volume, business volume expressed, affected condition types, entered net invoice, including service management
Related Notes : 
       
| 96375 | Error w/ vendor business volume from subsequent debit/credit | 
| 96137 | |
| 40147 | Collective note: Subsequent settlement (Purch.) Release 3.0 |