It really depends. If the contract gives ownership of the work created to the purchaser, he has no rights to it whatsoever. Moreover, trying to do a clean room implementation of your own code is almost impossible without help. A permissive license would give the purchaser unlimited use of the product, including resale while still allowing the producer unlimited use, as well. If the contract is written correctly, the producer might even retain ownership, with the right to use different licenses, while the purchaser would have few or no restrictions.