I believe what Leonardo is trying to say, for a multiple line(he means, a lot of multiple lines) PR/PO, they want to post a FM document(commitment update) on the grand total and not update the FM on line item basis - PR/PO->GR->INV->Payment! As they do not know when all of those purchases will be done, but they are sure that it will be purchased!.
I believe he wants to enhance the PR/PO document with a tab input of their grand total, guess for the whole year, for commitment update.
If that is the case, Leonardo, apart from what Eli said, SAP is a tightly integrated product. One thing leads to another. The Architecture and the business process of MM/FI/FM has been built in such a way that commitment is updated when its consumed and expensed!
I hope it adds clarity not only to the solution but also to what you are trying to say.
Cheers,
Nathan