invoiced for device returned

Worst service ever, I app**** for a device then after a month thats then I received it only to find out it was the right product yet wrong memory of the device. I had to return it the following day yet I called in to cancel my contract n was told it was cancelled only to receive an invoice statement from telkom to be billed on device that I never opened n used yet been calling to cancel n being told it was cancelled.