
If there is no mention in the contract or "bill of sale" then there is no need for you to allow him to be used at stud. Once the sale is completed and you have the KC registration papers, there is nothing the breeder can do to make you allow her to mate a bitch to your dog.
Any arrangement like this with a pedigree dog should be registered with the KC incase of later problems/disagreements. You should if you decide to have the dog with the new agreement in place insist that he is only used if he passes all health checks and the bitches he mates do so as well. I have had an arrangement like this registered with the KC, really it is for bitches on breeding terms, but I have used it in the past to register an agreement like this.
If you with draw from the sale will you get your money back? You should be paid back as you were not made aware of the new conditions before you paid your deposit.