Paying for their Mistakes and Lies

So I have managed to settle my old West Bank account and recieved a settlement quote and paid the exact amount into their bank account. Recieved a few smses telling me thanks for the payment and that I have settled.

2 weeks after I receive a call from a guy named Ruben P. telling me that I am R360 outstanding and what not. Told him that ill send hom all my documents as proof of payment, with settlement quote and pictures of the smses i received. I did so and he phoned me telling me they made a mistake and I must pay. I feel that its their fault not myne and also If they do this now whats not to say they are getting numbers from their thumbs and doing it to others. I am very unhappy and feel that West Bank and Hammond Pole Attorneys is at fault here and not me as I only did as they said on paper.