Solution : https://service.sap.com/sap/support/notes/378398 (SAP Service marketplace login required)
Key words : 
update open billing document values, additional key words credit, credit update results, credit values, open receivables, 'real' update, billing plans, billing plan, credit limit, payment requests
Related Notes : 
       
| 425523 | Collection of consulting notes: Credit update | 
| 393937 | Update of credit value in billing plan | 
| 377165 | Update open credit values for credit management | 
| 360478 | Credit managemnt, billing plan, down paymnt request. | 
| 153741 | Open values are changed in the case of down payments |