If the agreement is that she gets a pup, then she gets a pup. That was her price, and you agreed to it. She provided a product/service in terms of her dog covering yours, and her fee for service was a pup or the value of a pup. An unsuccessful mating doesn't negate the fact that she provided the service. Unless her dog didn't cover yours (i'm assuming mating was witnessed) or you can prove that the "fault" for small litter was her dog (limited sperm or similar) then there is no reason she should be penalised. If the agreement had been "x percent of the sale price of the pups" or something which made her payment contingent on pup numbers and sale, maybe it would be different - but her price was either a pup from the litter, or the sale price of a pup. if your dog only has one pup and you want to keep it, then the cost becomes a dollar one for you to pay for the services provided.