AT&T customer service agent falsely sold me the phone (Buy one get one) - AT&T accepted their mistake but does not give me the due credit

I've been a AT&T customer for the past 15 years. In Aug 2018 I got a iPhone 8+ phone (buy one-get one). Only after doubling ensuring that I was eligible I accepted the offer. I have been calling AT&T customer service agents for the past 5-6 months to get this resolved but all my attempts have been futile. (Many a times the customer service agents hung up when they couldn't either comprehend or couldn't resolve. I can definitely say most of these agents are incapable to handle issues and do not have the audacity to escalate when its beyond their purview). Couple of these agents sympathized with my situation and transferred my call to the Loyalty department. I have been given case #s and assured that my issue will be resolved / taken care of. Neither have I received any calls nor has my issue been resolved till date. One of the Managers' empathized with my situation and gave a $350 credit after accepting that its AT&T's mistake. I still need to get $500 credit which AT&T is not giving. Managers from the Loyalty department have opened cases for this issue but without informing me the status the cases have been closed. I have called more than 30-40 times for getting my money. The AT&T store managers that I have spoken to have told me that AT&T is notoriously famous for this. How can I get my $500 credit that AT&T owes me?