what legal action can I take towards a credit union that attempted to pulled a "baith and switch" on me?

I was approved for an auto loan with my credit union. I was told that if I was buying used I would have a 3.25% interest rate for 60 mo. The next day I emailed the loan officer asking if year and mileage of the vehicle would have any effect on the loans length and interest rate and if so what the details were. He emailed me back stating that the vehicle needed to be 2004 or newer and must be purchased from a franchise dealership or private party. And that he had me down for 60 months at 3.25%. Based on that information I searched for a vehicle online, drove to a dealership and payed $105 for a pre-purchase inspection for a vehicle I was interested in. After negotiating the cost with the dealership I was ready to make the purchase and they sent a Purchase order to my credit union. When I arrived at the Credit Union. They would not honor the agreement that they originally told me and said I would now get 48 mo. at 3.5% or 60 mo. at 4% Even after I showed them the email I received before