Lied to about promotion

I can't believe I got scammed as well by this company. I recently switched over from Verizon after 20 plus years. What a mistake!!! Signed on my 5 lines with a promotion over the holidays. They offered $830 toward new iPhone 15 pro. I just checked my promotion credits for all my lines. They subtracted my trade in value of my phone off of the $830 promotion. The store made it seem that I was getting a new phone for nothing because I was led to believe I would receive $830 plus my $255 trade in. How does it make sense to deduct a trade in. 
That's like trading in a car and them adding the value of it to your overall costs. Makes no sense. Can someone please tell me how this is not a scam.