I sold my purebreed dog and now they want to bring him back. What are my legal obligations in the matter?

Around Christmas, I bought a purebred dog from a breeder. After a few weeks, I realized I didn’t have the time to properly care for him. I worked long hours and he had to be left in the crate 10-12 hours a day. He was just 5 months old and wasn’t completely housebroken. He was a very sweet dog but playful with a lot of energy. I wanted to find him a home with someone that had more time for him. Last month, a couple bought him. They paid cash and I gave them the dog with the papers. A couple weeks later, I emailed them to see how the puppy was adjusting. (They had another dog of the same breed.) All was fine . The dog was great . Today I got an email saying the dog was tearing up the linoleum on the floor causing monetary damage. They want to return him. I emailed- asked them to put an ad in the paper like I did. I gave them several phone numbers of interested parties who had contacted me. I recommended they crate the puppy while they were away from home. Not in position to take back.
The breeder is 900 miles away in another state. I bought him when I traveled to see my family over Christmas. The most logical thing seemed to try and find him a good home myself.