What happens if they made a mistake and I was given a rebate of thousand dollars I wasn't entitled to?

I purchased a jeep in August. All documents were signed and I took possession. The dealer contacted me. Now, they want their money and won’t release my title. I tried to get a duplicate copy of my registration through an auto tag company but the results come up no record. I called the jeep already and placed a complaint because they are harassing me. I plan to contact the state police and the attorney general for advice.