is there a way to control when an app snapshots a field?

I have not thought this all the way through, but when the record is in Add mode it can have a look up of the current total penalty, which obviously would not include the amount on the as yet unsaved record.

You can have a form rule to capture that value in a data entry type field whoever that data entry type field is blank. Ie as soon as the form loads, you grab the total penalty and it will be before the record is included.