On the Cancel Fee/Fine modal, change the Reason for cancellation label to Additional information for staff (see attached screen mock-up Rename-reason-for-cancellation.JPG for details). It will remain a required field. This change will make it consistent with the how the Pay Fee/Fine, Waive Fee/Fine and Transfer Fee/Fine modals work. I am not sure where the Reason for cancellation is stored on the 'action' record, but the renamed 'Additional staff information' should be stored in the same manner as it is for payments, waives and transfers.
Currently Reason for cancellation displays as Additional information for staff on Fee/Fine Details. Let me explain...
If you look at attached screen print Example-screen-print1.jpg you will see that I have entered 'Charged wrong patron' as my Reason for cancellation. If you look at the Fee/Fine Details for this fee/fine (see attached screen print Example-screen-print2.jpg) you will see that 'Charged wrong patron' appears in the Additional information column as STAFF (which is Additional information for staff). This should continue.