Misinformed by retentions agent regarding billing amount

My contract was due to end 08 June 2018. Phoned your retentions department 08 May 2018 to cancel and move over to prepaid due to financial constraints. Retentions agent convinced me to stay on same contract month to month but only pay R100 per month as have now paid off handset. Checked with her 3 times about the biling and was informed that the last bill of R271 would be at the end of May and then it would be reduced to R100. Got the bill for June 2018 - still R271. Phoned retentions department and was informed agent made a mistake and there is no way i could be billed less in June as my contract ran till 8 June 2018. She then embarked on trying to sell me all kind of other contracts which was annoying and i ended up putting the phone down. Can someone please listen to my query complaint . I want it stated in writing when i will be billed what as you have already **** to me. I feel that the agent i spoke to on 08 May should be liable for what she told me. This is not acceptable.