Bad service ever

On the 20/10/2020 I enquire at about cancelling my contract and I asked how much i pay. They gave me the cost of which it was R408 as i need to pay that then automatically my contract will be on prepaid after that. On the 11/11/2020 I send my cancellation with proof of payment . After a day i got an sms that my account has be converted to Prepaid account. Then on the 15th December i got email statement stating that I owe R504. I have a question to my self how am I owing them money because I called i was told that I owe 408. where does this money come from. I send twitter message to Vodacom Inbox looking for explanation a person replying my message he or she never answer my query. Now on my credit Score I owe Vodacom . for 3 months how because I cancel the contract with them my phone is on Prepaid because i can't afford it . Why my credit score has been mess up. Account number: I***-1 can someone have a look on this