Contracts entered into by reclamation districts authorized under this chapter for construction or for services or materials, may provide that payments shall be made in such monthly amounts or in such monthly proportion of the contract price as the board shall determine as the work progresses or as the services or materials are furnished on monthly estimates of the value thereof approved by the board; PROVIDED, That at least ten percent of each of the monthly estimates shall be retained until the contract is completed and its completion approved by the district board.