If you answer in the negative, expect to be educated at great length if this is an applicable subject. I do this to observe how well mannered the children are. If they're obnoxious brats who never listen to their parents, I won't sell this family a puppy. I, as do most reputable breeders, employ a contract as a means of setting down the agreement and guarantees of sale. This is a protection for both the breeder and the buyer.
Many breeders have a spay or neuter clause in their contract for pets.
Don't be alarmed, it does not mean their is anything wrong with the health or temperament of the dog. The dog being sold as a pet may just have a conformation fault such as the bite may be off a bit. Requiring the dog to be spayed or neutered is just a way of controlling the many accidental or misguided breedings and helps with the over population of unwanted dogs. After several months have passed, no one is going to be able to remember every little detail that has been discussed and agreed to and a contract is the best way of keeping the record. Please don't be put off when a breeder tenders a contract.