VVM lies!!!

I had finalised payment on a debt we had. I called in the week before so I could get the final amount as I knew it would be a bit more than the debit order. I was told I could pay the extra via EFT and when the payment reflects I could get a letter to state that our account was paid up. I knew exactly what the amount was and I received an sms as to final payment due. However when I called to ask for the letter stating account was settled, all of a sudden there was another balance of an extra R1000 which is absolute nonsense!!! How can you tell me this is the final amount, and from correspondence and calculations( which added interest) the amount is correct and all of a sudden want more??? That is just absolute ********** and I refuse to pay them any more money.