Hi,
You have to be careful when you change financial transaction for commitment item. Naturally, 90/1 might not require fund assignment, as no FM document is created for the line in question.
For business partner line logic, please, refer to note: 686383 - FM integration in Contract Accounts Receivable and Payable: Restrictions
Also, note 1854176 - FKKFMCHK_CUSTOM 005: Change update logic for FM and GM? See
SAP Note 1854176 could be useful for you.
Regards,
Eli