You can threaten to resign/quit. If there were a lot of you in your group who are in a similar situation as you ( who aren't partner yet) together you as a group may have more leverage. But in your case probably not much, since one person leaving may not be enough to significantly affect patient care and may not significantly change the valuation of the buyout. If you want to be more sure, would have an attorney look over your contract and the who situation to see if you have any legal recourse in your state. Though in the end you may just have to leave to cut your losses.