Is the discount that a contractor receives due to his relationship and experience regarding the price of building materials or services his to keep, or does it belong to the project owner?

In this transaction, the contractor acts as an agent for the owner, and all contractual rights revert to the principal. Therefore, if the buyer gives a gift to the agent, or if the price is reduced, this right belongs to the principal. This is because the gift or reduction is a consequence of the sale and is considered an increment to the price attached to it.