Account Mismanagement

I am extremely upset regarding the way African Bank does things. I have been a client for a very long time now and have always conducted my account correctly and on time. About 3 months ago i started a new job and i now get my salary later than i normally used to. What i did then was phone them to move my debit order dates, to ensure i have sufficient funds in the account to go off. My normal debit order date was the 25th of each month. They kept on taking it on this date, regardless of me making arrangements. But i still made sure there was money in the account for them to go off. Last month, they pushed the debit order through on the 23rd!!! There was no money in the account so my debit order returned. I NEVER GAVE CONSENT FOR THEM TO TAKE IT THAT EARLY! Because i know there wouldn't be enough money in the account. And not to even mention...THEY DEBITED ME TWICE! And never refunded the double debit. But this is not my problem, my actual problem is that i app**** for finance for a new car, now every bank is declining me because of this returned debit!! I phoned to ask them for a letter to state that it was their fault. \We don't have such a letter!"Now I'm stuck with a bad name!"""