Telkom must give up as Network Provider

I cancelled my contract after I had to pay R731 for a subscription of R339. I cancelled it beginning of September and they turned the simcard off immediately, while it was still full of data. The I communicated with them since I used the SIM card up to the 10th of September now in October they are sending me an invoice that I owe R520. For what since I called and paid them extra!