Solution : https://service.sap.com/sap/support/notes/83771 (SAP Service marketplace login required)
Key words : 
additional key words subsequent settlement, adjust purchase orders automatically, volume-based rebate, attached report 'zrahm', vendor business volume, functions -> --> additional data', create outline agreements, outline agreements created, additional data, subsequent settlement
Related Notes : 
       
| 40147 | Collective note: Subsequent settlement (Purch.) Release 3.0 |