Hi,
According to your description, no only FM is active, but also its integration with CO module, as KB15N creates internal CO documents. So, first of all, check if it is on purpose. If the answer is positive, and FM-CO integration is required in your project, then FMDERIVE is your answer. You should derive a dummy commitment item for relevant accounts: a rule, that you can base on business transaction, for example.
There is no possibility to 'skip' FM assignment: it has to be done for every relevant operation, But, as I said before, it can be defined as 'dummy' one.
Regards,
Eli