Solution : https://service.sap.com/sap/support/notes/111211 (SAP Service marketplace login required)
Key words : 
payments update negative open delivery, additional key words credit limit, incorrect credit data, credit exposure, symptom billed, billing plan, s067-olikw, program error, advance correction, program rvkred77
Related Notes : 
       
| 153741 | Open values are changed in the case of down payments | 
| 133156 | Credit exposure is incorrect for down payments | 
| 113347 | Down payment process. as of Release 4.0x (standard) |