Solution : https://service.sap.com/sap/support/notes/582900 (SAP Service marketplace login required)
Key words : 
system sets field vbrp-posar, billing status 'completely processed', field vbrp-posar, system incorrectly assumes, attached advance correction, terms milestone billing, fplt-fksaf = 'c', subsequent billing documents, milestone billing plan, milestone billing exists
Related Notes : 
       
| 204130 | SIS update: Collective note | 
| 196467 | Item type is not transferred to FI/CO interface | 
| 174141 | SIS: Collective note - Customizing | 
| 161862 | Multp.quant.transf. to CO/statistics/canc.bill.docs | 
| 158877 | Down payment requ.prev.quant.transf. to CO/statcs. | 
| 137148 | SIS reorgnzn:Billing doc., down payment clearings | 
| 123534 | SIS: Quantity update for milestone invoices | 
| 113347 | Down payment process. as of Release 4.0x (standard) |