Hi,
Your requirement is very tricky to handle, as SBO allows several ways of user to update the field.
Aside from your event and one pointed out by Ankit Chauhan
I suggest you handle also Click/Right Click event as User can cut and paste into this field.
Another thing is, if you double click on the row number, a new screen will come up. This will allow the user to input the amount here. Thus, you will need to handle this also.
Regards
Edy