Data disappearing

Two years ago I took a vodacom data contract, by the time I was about to finish it and tried to discontinue with my contract, the consultant convinced me to stay and rather have R85 flexi. So December I decided to upgrade data contract and took the one of 400G seeing that FNB used to give me 500G which lasted the entire month. With vodacom, I was previously using 100G day and 100G night and half the time it would last at least towards the end of the month around the 24th on day data. What I find quite funny with this 400G by the 12th, the day data was already finished and I called the customer service and since well they couldn't give me clarity on why the data disappeared so quickly, they offered me an airtime of R165 so I can use it to purchase data. Our agreement with vodacom is that they will debit R399 but when the statement arrives, they say that I'm owing R450 of which I don't know where the R50 came from, so I'm wondering if we as vodacom customers, are we the ones who have become a casualty on this matter of the man that vodacom had to pay billions to, we are helping them with these extra amounts which we don't know where they have been picked from. I just want to cancel this contract.