Hi Szymon,
According to me, whatever the substitution you might use (it may be from CO or FI in this case), system only considers substitution rule ahead of OKB9 settings. As Ajay sir rightly said, OKB9 settings are considered only if we don't maintain anything manual or any substitution rule only.
@ Kami:
Please go ahead as Ajay suggested and please let us also know the results
Regards...
Jose