In my customizing I have selected to carry forward the budget associated and also the document chain (chain carry-forward). I mean, this was customized in the client like 7 years ago.
Now, in a PO (or FR) carry forward process, it is clear that the 'budget' carried forward should be the same as the "commitment" carried forward. The system in this client creates differences between the budget and the commitment. It is an error if they are not equal.