Solution : https://service.sap.com/sap/support/notes/151533 (SAP Service marketplace login required)
Key words : 
online service system hotline, attached user exit solution, absolutely require billing-relevance, user exit solutions, tvap-fkrel = 'f', tvap-fkrel = 'b', import support packages, unexpected side-effects, internal table fxvbup, partial quantity occurs
Related Notes : 
       
| 1284106 | Missing user exit for changing item status | 
| 387890 | Open delivery value with third-party and partial delivery | 
| 210500 | Billing status of a third-party item | 
| 170183 |