Unfair, poor service.

In September last year I agreed to make TWO instalment payments of R475 for the Wonga loan I took out, one end of September and the last one end of October. I was given a discount due to my circumstances. They managed to deduct the first instalment on time in September but in October they didn't deduct on the agreed date. I got calls and agreed to pay my final instalment end of November. I was promised a paid up letter after my final payment yet I have not received it. I got another call from them saying I still owe over R300 because THEY failed to deduct on time in October. They are making the fact that their system failed them, my problem which is not fair. All the conversations are on a recorded line yet everytime I speak to someone, it's like they don't know what I'm talking about. I keep getting promises that a manager will be in touch or help sort it out but nothing changes. I can't be penalized for something THEY failed to do!!!