Bait and switch AT&T

I bought a new Samsung for $1,200 and sent in my old Samsung for an $800 credit. I signed up for the $50 a month plan. I had not received a credit as of 4 months so I called. I was told to wait a few more months. At 6 months, and still no credit, I called again. I was told the promotion for the $800 credit for my old phone was only for the monthly plans costing $65 and up. If I wanted my $800 I had to switch. THAT'S BAIT AND SWITCH. The supervisor said he could authorize $400 but no more. The trade in department told me they'd give me the $800 but only if I switched to the $65 a month plan for 36 months and they'd divide up my credit over that time. That's against the law. I'm writing corporate and will see what happens. I have a friend of a friend I just met who is fighting the same thing with AT&T so this is no error. This is intentional and illegal!!!