Solution : https://service.sap.com/sap/support/notes/53201 (SAP Service marketplace login required)
Key words : 
additional key words credit limit vkm1, releasing sales orders, decides internally, decided generally, include mv45afzf, availability check, prerequisites solution, availability, prerequisites, note 52272
Related Notes : 
       
| 1324433 | Availability check during credit block release | 
| 52272 | Availability check during release | 
| 47469 | Credit release prod.sel. missing confirmed qnty | 
| 36429 | 3.0C: No purchase order req despite credit release | 
| 35913 | Credit limit and block and purchase requisitions | 
| 33103 | Credit check, availability check, performance | 
| 33078 | Credit check: Blocked documents are approved | 
| 31248 | Release, another check and block, performance |