Solution : https://service.sap.com/sap/support/notes/201821 (SAP Service marketplace login required)
Key words : 
additional key words billing plan, user exit mv45afzz userexit_pricing_prepare_tkomp, attached source code, received remains fixed, periodic billing plan, periodic billing plans, group condition indicator, billing plan, milestone billing, group condition
Related Notes : 
       
| 381348 | Using user exit, customer exit, VOFM in SD | 
| 170183 |