Is it unlawful if a used car dealership will only help me pay a portion of a repair if I sign an agreement stating that I will not publish any comment

I recently bought a 2018 Hyundai vehicle at a used car dealership. Days later I noted that there was a major issue with the vehicle and after a very long discussion with the managers I was told by the customer service manager that I could fix my vehicle at my local Hyundai Certified Service Center and that they would cover up to $500 in repairs. All I had to do was bring back the invoice and they would write a check. At the time, she did not mention any exclusions or anything that would not be covered as long that the repair stayed below the $500 range. After fixing the car and submitting the invoice they are now saying that they will only cover labor costs which was only about $320 and will not cover the cost of the parts. And in order for me to get the $320 I would have to sign an agreement stating that I would not publish any negative remarks about the dealership. I don't know the legality of their actions and I hope someone is able to provide me some guidance. Thanks in advance!