How would I go about getting the money or the car back from my ex girlfriend and can fraud also be used in my case?

My ex girlfriend sent me a text a few months back saying she was going to have to file with the court for child support against me. When I called her I asked her why she said she needed a car cuz she couldn't afford one on her and her new husbands salary. We share a 6 year old son but we ve been separated since he was 2. I didn't want to see her not have a car for my sons sake so I told her I would buy her a car. At which point she made an agreement to me that if I buy her the car she won't take me for child support. I agreed but told her if she did decide to take me to court then she would have to return the car to me or pay me for the car. She agreed and I purchased her the car a week later. About a month ago I received papers to go to court and went last week at which time she got support from me. Now I ve ask for the car or the money and she refuses to pay me or return the car. I was told that I should include fraud in this case cuz she made an agreement that wasn't hers to make.