Where do you live? There was an interesting case in New York City a few years ago that involved the same issue, a person paying more for a "teacup" Maltese who ended up weighing eight pounds.
O'Rourke v. American Kennels
In this case the breeder was found to be in violation of Uniform Commercial Code because she mislead the buyer into believing she was getting a smaller "teacup" dog and charged $1,000 more for it. The owner was awarded the $1,000 difference in price by the court.