Trade-in Issues

I sent my phone for a trade-in on last November and received a notification from the carrier(UPS) that it was delivered. 

However, my account was showing my trade-in was not received. So I called the customer serv dep to make sure everything was in order. An agent on the phone confirmed all the details and informed me that my trade-in was received and evaluated. Thus, my account will be credited in amount of $800 for the trade-in.

About a month later, I found out that credit on my account disappeared. So I called the customer service department again. An agent on the phone could not explain to me why my credit disappeared, but assured me he will credit my account for the full amount. Unfortunately, I am still waiting for the credit. 

It has been almost 3 months since I sent my phone for the trade-in and honestly, I am exhausted dealing with this. I am contemplating whether I should just ask Verizon to sent my phone back.

If anyone has had a similar issue, please advise. Thank you for reading a rather long post.