Mobile Device Trade-In Discrepancy

Hello, I'm running into a brick wall with Xfinity regarding my device promo credit. I began new service with Xfinity in September, where they were offering a $500 device trade-in credit. I sent in my device in late September, then I received an email from Xfinity a couple of weeks later saying that they received my device, and that the promo credit would be applied to my account. Fast forward a couple of months, still no device credit, so I reached out to find out what's going on. They created a ticket that went unanswered for over a month (at least to me as the customer). I went into an Xfinity store on 2 Jan, only to be told that the ticket was closed, and that the device promo credit was cancelled because I provided the wrong serial number. I then asked why they sent me the email stating the credit was going to be applied? Why wasn't my old phone sent back? This doesn't make any sense and seems like a bait-and-switch. If there was a slim chance I provided an incorrect s/n #, they should've sent my device back immediately, not wait months to play this game. To this day, I still don't have my old phone or the promo credit, beyond frustrated.