First National Bank
Incorectly Deduction of debit orde

I received an sms and email from FNB stating that my there were less than R250.00 in my account which was incorrect. I called them and they apologised for the mistake they have made. Two days later I checked my balance and see that there were an amount of -R1500 less in my account. I checked my statements and realised that FNB deducted that amount from my account. I called them and they told me that as soon as money comes into your account then they can deduct. My issue is how can FNB just do what they want as we have a written agreement for the debit order to go off on the 25th of each month and not on the 22 of the month and another issue I have is, no money was deposited into the account only the money that was in the account was part of my balance. I didn't even receive an sms or email to state there were deductions made from my account