2109. Where the price is fixed by the contract, the client shall pay the price agreed, and may not claim a reduction of the price on the ground that the work or service required less effort or cost less than had been foreseen.
Similarly, the contractor or the provider of services may not claim an increase of the price for the opposite reason.
Unless otherwise agreed by the parties, the price fixed by the contract remains unchanged notwithstanding any modification of the original terms and conditions of performance.