Trade-In iPhone Promo - Did not receive what was promised

On 12/17/20 I went into the retail store. Associate told me they were having a trade-in promotion. It’s the 2020 Apple Trade P28 promotion. I asked her how much credit I would receive if I traded in my iPhone 8 64gb for an iPhone 12 64gb, she checked her iPad, and told me I would receive $415 and that I would be receiving $90 of it day of. I thought this was an excellent promotion so I did it. I paid the tax, received the $90 credit and was told the remaining of the $415 would be credited through 30 months. A week later I checked my account and saw I was only receiving total $293 total credit ($7 monthly credit for 29 months), not the $415 promised. When I called T-Mobile Customer Service, they said the promotion was UP TO $415 and depended on the condition of your phone. The retail store associate DID NOT say “up to,” she said SPECIFICALLY $415 exactly and she didn’t mention anything about the “condition of your phone.” Customer service associate said I would receive a call from a manager within 24-48 hours, no response. I have been a customer for 18 years and want to receive what I was promised which is the $415 trade-in promotion credit. I am tempted to go into the retail store and file a complain, but feel I need to go to someone higher to get my issue resolved. Please help. Thank you!