Bank charges incurred because of debit orders before agreed date

This institution debits my account before the debit order date and when i phone them they are telling me they are not going to refund me the bank charges incurred. And our arrangement is that they debit my account on the date i get paid and if it falls on a sunday they must debit on a friday however thay have a tendency of debiting on thursday. Now they made my bank to charge me R901 for bank charges for unpaid debits and they are telling me they wont refund me. This is unfair how do they expect clients to pay off their debts while they are busy drowning them to even more debts.