I'm not sure I understand what the problem is. For a library system that includes fining its patrons, I would create a database that tracks when the patron checks out an item and when the item is checked back in. After the item is checked in, it would see if the item was late and, if so, then add a fine to the "fine" table. That would require either an append query or vba.

Another approach would be to just record the check out date and check in date and always calculate the fine that is due instead of explicitly recording the fine that is due. To do that, you would simply create a query to calculate the difference in days between the check in date and check out date and calculate the fine based on those two days.