A dentist office made me believe my insurance was going to cover more, and they knew I had now they want more money, are they in the fault.

My insurance policy allows me a $1000 a year that they will cover. I used $400 earlier in the year and had about $600 left but my dentist made me to believe that I had my entire $1000 still, even though they had called my insurance company and knew I only had $600 left and made every quote and bill that stated I had $1000 worth of coverage. This made me think that the $400 was from the year before and I actually had $1000 for the new year. Now they want another $500 that my insurance wouldn't cover, I asked if my insurance would cover two extra teeth removals they insisted that I would be.