How can I make dentist give me a refund?

[Melycast3]

I went to this specialist for a dental implant. Back then I had no insurance so they charged me $5900 for all the work that I needed, the implant, bone graft, gum graft, abutment and crown. I paid for the whole thing using Care Credit. When my insurance kicked in I changed dentists to one that is in network (so that I could have the crown done there and the insurance pay) and now I cannot get them to refund me for the abutment and the crown which were never done. They charged me $650 for the abutment and $1750 for the crown and they sent me an email telling me that I need to sign a form saying that all I will be refunded is $1500 and that's it. What other steps can I take? I already contacted Care Credit and they will file a dispute but they told me that they could not guarantee anything. That dentist is basically stealing, they are keeping money that isn't theirs. I lost my tooth because of a very badly abscessed root canal and the implant process and healing and bone and gum graft have been a traumatizing experience and on top of that I now have to deal with the dentist keeping my money. I am so upset.