Solution : https://service.sap.com/sap/support/notes/640442 (SAP Service marketplace login required)
Key words : 
availability control incorrectly reports budget overruns, large commitments documents requires, type = fmioi-rfpos pass, austrian government agencies, function module fm_open_item_read, fiscal year change, ledger change reason, payment budget, commitment budget, edit documents
Related Notes : 
       
| 716898 | Unexpected update of sales order in FM | 
| 678397 | Performance improvement in commitment update | 
| 669928 | FM irrelevant PO line item updates FM |