Debits on the wrong day and still debit when you are done paying

They debited the last money I had in my account when I was supposed to use it on other important things on the wrong date (not my pay day). When I called, they claim that the contract allows them to debit on a Friday. This was supposed to be my last installment, but already one of their agents called to make an arrangement. So, they say they will still debit even though I am done paying, so that I later get a refund. I never had a problem with them in the beginning. So frustrated!