Dissatisfied

I took out a 24month data contract with cell c in May 2018. I was given pro-rata amount of data and an amount to pay by the end of that month. From there on the agreed amount was devoted from my bank account. I cancelled the contract a month prior to it expiring and was given a pro-rata amount of data as the contract expired on the 5th May 2020. My account was debited with the full amount. I queried this on several conversations with different consultants and every time was given a different explanation to why I'm paying the full amount. I received my statement via email stating that an amount of R451. 43 was in the minus. To my understanding that was owed to me as I had only received the pro-rata usage of data as the same as my first month of the contract. I contacted cellc again and was told to contact other departments. I need a supervisor to explain why the statement states that my account is in the minus and why I paid a full month on the contract if I was only given 5 days of data