Proof of closed account

I had an account with Absa bank, it was on a negative balance. I paid off the balanced and asked for Absa to close the account. Now I need them to give me proof of account closure as I need to hand it in at work because I am changing my banking details. So to my surprise, Absa is not able to give me the proof of that my account is closed, why. I mean I do not owe them a cent, all I need is a proof that my account is closed. I will not get paid this month if I don't have that letter, I have a family to feed, what is so difficult about giving me a proof of letter that states that my account is closed.