How to get a resolution? SCAM!

My parents recently upgraded their phones after a T-Mobile employee informed us they were eligible for an upgrade with an $830 refund per line, plus a Costco gift card on our existing plan. Two months later, I checked and found that we're only receiving a $300 credit per line. When I followed up, I was told our phone plan doesn't actually qualify for the higher credit, and now we're facing an additional $60 per month to receive the credit we were promised.

The employee acknowledged his mistake and even offered to pay the difference himself, but his manager reprimanded him for saying that. Rather than taking responsibility or addressing the issue, it seemed like the manager wanted him to cover it up. I've been going back and forth between customer service and the store, but nothing has been resolved. I also emailed the manager requesting the regional manager's contact information, but I still haven't received a response.