Xfinity took my phone as a trade in and refuses to give me the $830 credit I am due since April 2025!

In April, my son took advantage of an Xfinity promotion and traded in his phone for a new one.  Well, we were TAKEN ADVANTAGE OF.. He went to the Concord, NH Xfinity store and they gave him the envelope and mailing label to send his new on back.  In June, I received the phone at my home with no explanation just the phone.  I went to the Nashua, NH Xfinity and they said it was returned because he did not clean the phone on his wifi which he was not told it had to be on his wifi.  The associate gave me instructions and a new mailing label to send the phone back in the box it came in.  It has been a nightmare ever since.  Since June, I have had to call every month and they would give me a credit.  I spend at least 1 to 2 hrs monthly explaining my situation getting transferred to 4-6 associates that do not no what they are doing.  I've even been transferred twice to the Spanish speaking customer service.. I DO NOT SPEAK SPANISH.. I called the end of October and spoke with a mobile billing department supervisor and was told they would manually do it and I would not have to call anymore.  That did NOT happen.  I did not receive a credit and had to call again and was told that was not true so I continue to call.. Then December 1st I called and was told I would receive a check for the difference of the trade in value of $830 and what I was already credited $622.90.  I have continued to call because I HAVE NOT RECEIVED A CHECK.. I called on December 17th and was told I would have a check the next day in my mailbox he even gave me a ticket number... GUESS WHAT NO CHECK.. I called today and spoke to a supervisor Maya in the Billing Department and she told me that I was not getting a check or credit and my phone would not be returned to me because they didn't receive the traded in phone back until AUGUST..  I really wish someone that knows what they are doing would read this and help me because I am reporting this to the Better Business Bureau as well.   Xfinity TOOK MY SON'S PHONE AND IS NOW REFUSING TO GIVE IT BACK OR HONOR THE TRADE IN CREDIT THAT HAS BEEN GOING ON SINCE APRIL!! I would like this issue resolved IMMEDIATELY.  Please do not respond to this unless you actually know what are are doing and can resolve this issue IMMEDIATELY!  Thank you for listening.