Unfortunately you cannot legally force a person from whom you bought something from to give you a receipt for your purchase where in your case you paid cash for a dog.One way to get a receipt is simply to send a letter to the seller memorializing the cash transaction and the seller's failure and refusal to give you the requested receipt for the payment of the dog. Keep a copy of the letter for future reference and need. It has become evidence of a receipt for the purchase of the dog.