Billing

Purchased a mobile phone plan a few months ago with a rep at Holyoke Mall store. He said I would be getting $40 off my monthly bill for 12 months. I specifically asked that he was sure I would be getting the credit for 12 months them after that bill would be about $59 a month. He said yes so I did the deal.I asked for paperwork he said it would be on line. 

When I got my first bill notice it didn't show discount so called billing support. The rep looked up my account and gave me the $40 credit and said I shouldn't have a problem going forward.  The next bill still showed $59.14 so I called again. This time a different rep said he could give me the $40 credit but I didn't qualify for the deal because I was on Internet Essential. I explained again what I was sold at the store. He said I would have to go back to the store. So I did. Talked to the same person who sold me the phone. He denied saying that to me so I said to take the phone back and cancel my plan.I told him I didn't lie about stuff like this and distinctly remember our conversation. I spent over 3 hrs at the store with that rep, the manager of the store and the virtual assistant due to it being over 14 days allowed to cancel. I said if I wasn't told by the first rep on the phone I was all set I would have gone back to the store sooner . The manager finally agreed to take the phone back. The rep told me to go get the phone number transferred to different carrier then bring it back to him personally. I did that the same day. He reset the phone to factory settings in front of me and said I was all set. I got an email yesterday saying I owed $80.50. I called customer support today.. They said the system showed no phone being returned and I needed to go back to the store. So today went back to Holyoke Mall store and spoke to a Lisa as they couldn't give me the rep who sold me the phone. I explained everything again and said I wanted to talk to manager. The manager wouldn't see me but did tell her the phone was returned under " an unsolicited sale" and would take up to 2 months to show in the system. I told Lisa that I wouldn't be paying the bill as is until I got a  $40 credit on Sept and Oct. Billing is adjusted to show one day usage. What else do I have to do to get this resolved and have the billing put on hold so it doesn't affect my credit. I am thoroughly discussed with the way I was treated by both the manager and original rep.Please advise. Thank you.