Withholding Refund By Car Dealership

I had paid a booking amount and a down payment to a car dealer for purchasing a car. The dealer as I learned later is not authorized to sell that particular car and informed me that he cannot get me the vehicle I wanted. When I asked for a refund, he agreed to give a refund but it now not honoring his commitment and keeps delaying re-payment of money. I want to send a legal notice to him for getting my money back and seek advice on the way forward.