If you are the seller of the dog, and the buyer backed out of the agreement without a reasonable cause, you could take them to court (probably small claims) to address their breach. Either they have the dog and you get the money, or alternatively, they bring the dog back and you have to decide what you want to do. The Uniform Commercial Code as adopted in WA is found in RCW Title 62 and it governs what happens when contracts are breached.
Sounds like what you have is a slightly older puppy, and your ability to sell the puppy to another buyer for the same price might be compromised. On the other hand, if you are in this business because you love your dogs, it might be better long-term business to take the dog back and thank goodness he didn't wind up in a home where he would not be appreciated, and find a better buyer. Only you know your situation.
Your statute of limitations if you have a writing is six years to file suit; if there is no writing, then you have to file suit within three years. Hope this helps. Elizabeth Powell
I agree with my colleague. What your rights are depends on who has the dog and how far you want to go to enforce the agreement. In addition, you need to think about the dog - do you really want one of your dog's in this person's home?