Lied to about promotion

So same thing literally just happened. 

I went in to possibly trade in my phone for a promotion. Go from an iPhone 12 mini to a GooglePixel7. I was told my phone could trade in for $300 with the promotion and that it was in great condition. 

So I went for the trade. But at the time of the deal, the acceptance offer was lot less than what the guy told me: $140. His words were "The trade would be adjusted when they get the phone in the mail." And he kept pressuring me to accept after I started to have doubts. But he seemed like he was telling the truth so I accepted.

It was a deferred trade and the gentleman said I'd have to mail it though I did not feel comfortable doing so. So a few days later I took it back to the store and had them send the phone in.

This morning I just got a message from T-Mobile about my trade in. It is apparently NOT in great condition (which is bogus) and is now worth $90 trade in. $210 less than what the guy told me. 

Called customer service to see about this issue and the person (Abby) was extremely condescending like "You don't understand how promotions work" "You already got the promotion." At which point I said I'd just go into the store and deal with them in person, then hung up. Now T-Mobile won't stop calling me and leaving lengthy voicemails saying I don't know the promotion, etc. 

I cannot afford the phone I have without the promotion I was told about. I should have just kept my iPhone 12 mini. Considering going to Verizon since I have internet with them.