Lexus of North Miami

I recently returned a Gx 460 and purchased a used car. After negotiating with me they made me believe that they had handed me the best deal possible. It was not until later that I realized they charged me $7000 for the car that I had returned and that $7000 was added to my new bill. Therefore now I am stuck with a very high bill that I can not even pay. They also changed the interest rate without even letting me know. I have been a customer with them for some time now and I am really unsatisfied with the deal they said would be the best for me.