Hi Murtuza,
If a custom FM is attached to Event 1330, it will only call that event. If you need to call your custom logic only at some specific payment term condition and rest should be standard, then as Bill suggested, you would have to use If/Else, Case/Endcase to incorporate the standard logic with custom logic.
E.g. in our project, for a specific payment term we populate the field e_faedn using custom logic, and for rest we use the standard code as it is.
Regards,
Ravi