Solution : https://service.sap.com/sap/support/notes/96224 (SAP Service marketplace login required)
Key words : 
additional key words subsequent settlement, 'supplementary vendor business volume' indicator, system generates message mn380, determined vendor business volumes, vendor business volume, vendor business volumes, subsequent settlement purchasing, volume-based rebate, po history records, purchasing document &1
Related Notes : 
       
| 40147 | Collective note: Subsequent settlement (Purch.) Release 3.0 |