I had a customer do this to me and I contacted consumer direct and asked their advice. They said that if the customer had simply changed their mind then keeping up to 10% of the sale price would be acceptable.
In my particular case I decided to give them a credit note against another car and they came back 3 months later and bought another car. Worked for me because I still got a deal and worked for the customer too because they didn't lose their money.
Maybe you could come to a similar arrangement with this dealer?