Honestly, I think your best bet may be to wave the contract at them and threaten, say that you want the dog back, and if they don't give it back you will sue... offer to refund their purchase price to them in return for the voided contract. It's called bluffing. No, your contract won't hold up in court. But if you threaten them AND then offer money for the dog AGAIN they may bend. If they don't, I think you are sadly out of luck.