Am I leglly responsible for purchasing this vehicle?

I was approved for an auto loan through my credit union. Given a check for the dealer. The credit union set up my payments and interest rate. They dealer sent in the check from my credit union and the CU denied itr because it did not meet the required "must be under 100,000 miles". I had the vehicle for 3 days and the dealer called and said they got me financed through another lender and I need to come down and sign the new contract. I do not want to go through another lender so I said I would broing the vehicle back and they said I couldn't do that. Am I still legally responsible?