7-3-1324. Alteration of contracts. When it becomes necessary in the opinion of the manager to make alterations or modifications in any contract entered into by the municipality, such alterations shall be made only when authorized by the commission upon the written recommendation of the manager. No such alteration shall be valid unless the new price to be paid for any supplies, material, or work under the altered or modified contract shall have been agreed upon in writing and signed by the contractor and the manager prior to such authorization by the commission.