What's my best recourse to follow up on being lied to by a dentist's office manager?

I'm not sure why they ured you you could receive free services, but if they did and the office manager admits it, and you relied on their urance to procure their services, then they have violated a verbal agreement between the two of you. If you signed a form that was contrary to the verbal urance, though, you are out of luck. I would write a letter explaining that the office manager said you would get free services and admits she made a mistake. Because of your mistake, you relied on her information to your detriment. This only works if you would not have gone to the office but for her urances. If you were going to go to that dentist anyway, you probably would have to pay for the services they rendered.