Hi Eli,
thanks a lot for your reply.
After reading "Example: Transferring Posting Data from Asset Accounting - Funds Management - SAP Library", I thought it would be more complicated than that 
(G/L which points out to different commitment items depending on the situation, commitment items which should be declared as statistical or not, etc.).
I am trying your simple rule then and hopefully it should be fine!
Regards,
Olfa