The contractors to whom such work is let shall enter into bond with said district, in such amount as the board of irrigation commissioners shall deem sufficient, for the faithful performance of their work; and said contractors shall be paid for their work as the work progresses, by the county treasurer, out of the funds in his hands set apart for that purpose, on orders drawn by the president and secretary of the irrigation district, in the same manner as provided for the purchase of such works: Provided, That said irrigation commissioners shall retain at least twenty percent of the contract price until the completion of said works, and until the said board of irrigation commissioners shall be satisfied that said contractor has fully complied with his contract.