Mixtelematix company

I have a tracking device with mixtelematix for many years. I sold my vehicle in November 2020 and asked mixtelematix for a settlement on 17 November 2020, they sent me the cancelation settlement of R300 which I paid on 23 November 2020 and emailed mixtelematix proof of payment. Mixtelematix emailed me confirmation of settlement. For two months all was well until end of February sent me a message that my account is in arrears. I cantacted them and they apologized to me and said its a mistake. March it happened again. I called them and they apologized to me once again. Today I have been handed over to an collection agent. I have proof of all what I'm am saying.