Need help resolving ~$600 Xfinity Mobile trade-in billing discrepancy

I need assistance from an Xfinity employee with an unresolved Xfinity Mobile promotional trade-in/billing issue.

I purchased a phone with an expected promotional trade-in credit of approximately $600. I shipped the trade-in phone within the required timeframe.

The device was initially rejected because Find My iPhone/anti-theft was still enabled. When we were notified, we followed the instructions and turned Find My iPhone off within the timeframe we were given. Despite correcting the issue, the phone was returned to us.

Assurant subsequently directed us to Xfinity to resolve the issue. After multiple contacts with Xfinity, I was told last week that the return/resubmission was approved and was sent new return information and a shipping label. However, the accompanying email continued to reference the original expired return deadline, so I did not want to send the phone back without confirmation that the approximately $600 promotional value would be honored.

I contacted Xfinity again today and received conflicting information about whether the trade-in can still be processed. My account has now been documented again and another escalation ticket was opened and subsequently marked completed.

At this point, I am simply asking Xfinity to resolve the approximately $600 discrepancy in my Mobile billing.

I am open to whichever resolution Xfinity determines is appropriate. If the trade-in can be reinstated, I still have the phone and can return it. Alternatively, I would be satisfied with Xfinity applying an equivalent monthly bill credit for the remaining promotional period without requiring the phone to be resubmitted.

I have screenshots, shipping/return documentation, prior case numbers, and the history of my contacts with both Xfinity and Assurant. I would appreciate an Xfinity employee reviewing the complete history and helping resolve the billing discrepancy rather than sending me back and forth between Xfinity and Assurant.