I don't know if it's legal, but the ethics are sketchy. Your state will have a medical board, or call the attorney general's office.

I think the legality of this will depend on the state. As far as I can see from Googling, federal law doesn't prohibit specialists from paying a referral fee back to a doctor, unless that specialist is providing a narrow set of services (it looks like medical testing and medical test interpretation).

I know with lawyers, referral fees are legal as long as you disclose it to your client, so they know you have a financial interest in the other person taking the case. Also, only lawyers can get referral fees for referring you to other lawyers, as they are bound by professional ethics to ensure you are not getting screwed, and there's a higher professional governing body that enforces such ethics. Maybe it's the same with doctors.