Well, it's actually 1875038 - Enhancement of FM Derivation Tool
But, what I still cannot understand is at what stage do you have this problem. If I get it correctly, your FM_MM commitment item is of nature 40, right? So, when is the issue raised?
Also, check note 1263965 - Incorrect GR/IR FM account assignment derivation for service; it might be relevant as well.
Eli