Randomly billed for a device I returned 1.5 years ago!!

I made a recent change to my Xfinity Mobile plan which was supposed to drop me from $90 to $80 per month. So that drop went through, but I also saw an unexpected charge of $175.11 on my next bill. When I looked into it, it was for a device I had returned when I started my plan because it was the wrong color. I received and email confirmation from Xfinity Mobile on 5/22/2024 indicating they had received the device. I figured the matter was closed and the billing after that was correct until my last bill. It seems like the phone was never removed from my account, and since it was a free phone, I was getting a monthly credit to offset the monthly charge just like the 3 actual phones I have on my account. I called a couple of weeks ago and after spending at least 30 minutes I was finally able to get the agent to tell me that they would credit the $175.11 in the next few days. Today, my bill is due and I still have not been credited. When I called again, I went through the same thing all over again, and this time the agent is saying they are unable to find a record of the return, so I should visit an Xfinity Store to get this resolved. I have no idea what the person at the store is going to do differently. So far, I have wasted almost 2 hours of my time trying to track this down, and now I have to make another trip to the store to try again????? This is is simply ridiculous!!!!!!!!!!!!!! Hopefully, someone on this forum can be of more help than the phone agents have been