Solution : https://service.sap.com/sap/support/notes/721326 (SAP Service marketplace login required)
Key words : 
changing ingrained posting procedures, 31 vendor   300-31 vendor   300-40 expense  600the, line items found increases, account statement document, direct cash flow, sample function 'flq_sample_reduce_2cl', sample function 'flq_sample_modify_inv', straight posting chains, customer exit functions, payment document works
Related Notes : 
       
| 848831 | Permitted changes in the exit for invoices | 
| 731562 | Liquidity calculation: Enhancements in plug-in 2004.1 | 
| 666327 | Extended reduction exit for invoices | 
| 614240 | FLQAD: Rules for the second step |