I don't know how things work in your country, but there may be restrictions placed on registering the puppies with the recognised kennel clubs, and the original breeder will often be able to view both registrations and the results of the health test that should be done before breeding. How far the contract can be enforced will also vary between countries. No matter whether or not the contract is enforceable in law, knowingly agreeing to it with the intent to break it is decidedly unethical.