Urgent Complaint: Missing Trade In Credit.

I upgraded to a new phone at the end of October. Mailed my old iPhone in to Assurant. Was promised at the store I would be receiving 700 dollars for the phone. I received an email confirming they received the phone and then a few days later offering me $500. I accepted this offer and have the email confirming this. Since then, Xfinity is refusing to credit my account. I have been on the phone with customer service (no help), chatted multiple times (get told answers that seem promising and then nothing happens) and went to the actual store yesterday and they said they can’t help. I have been told to call Assurant and then Assurant tells me to call Xfinity. There are a number of reasons why I am getting told the credit hasn’t been applied (wrong IMEI, haven’t received the phone, cracked screen, credit will be coming next month, technical difficulties, etc). This is the most frustrating experience and I need a resolution. Looks like others have had this problem and would love guidance on how to get this resolved.