T-Mobile Won't Credit Me Correctly for My Returned Phone. Help!

I moved two of my AT&T phones over to T-Mobile in March.  One of the phones is an iPhone XR.  The back of the phone was cracked but the front was flawless.  The T-Mobile guy at Sam's Club kiosk appraised the return value of the XR at $400.  He took the IMEI on the phone and logged it on the system.  After our new T-Mobile lines were activated, I sent out the XR using the pre-paid UPS package that they sent out.  Today, I received an email saying that the IMEI of the quoted device is different from the IMEI of the returned device.  The email said:

Looks like there was a mix up-the make, model or memory of your quoted device doesn't match what we received or it was damaged. We will adjust your trade-in value to $55 because of this difference.

This is so unacceptable and so unfair, because (1) I did not plug in the IMEI in the T-Mobile system.  I believe it was a typographical error on the rep's part that he plugged in the wrong number.  I remember that when he tried scanning the number with his company iPad, he couldn't successfully scan so he typed in the number manually, and; (2) The T-Mobile representative ocularly the device, removed the case and made sure that the screen was not damaged (although the back was).  And he found it acceptable with the return value of $400.  Please note that the T-Mobile website says “The screen on my device is free of cracks (Scratches and scuffs are acceptable)”.  Again, the screen was flawless.  It was the back that was cracked.

I am so upset with this that I want to switch back to AT&T.  I hope the T-MO people in this chat can offer us some relief for this unfair issue.

(And I haven’t yet even covered the dropped calls at my residence and place of work!)