Create a new Prepaid Invoice and leave the rest to ADI-diary.
As you book a lesson, the software looks for any invoices for the given pupil and if found,
debits the charge for the lesson, deducting it from the prepaid amount.
It also notifies you of the amount left in credit.
When the prepaid amount is exhausted, the software still books the lesson but no notification is displayed.
If a lesson is deleted, the prepaid amount will have an opening to book another lesson.
ADI-diary keeps track of such situations and the next time you book a lesson for this pupil
the software will deduct the correct amount.