(a) The District shall pay the contract price of such contracts as hereinafter provided:
(b) The directors shall draw a voucher on the District depository for the amount of any payments in favor of the contractor or his assignee. Said vouchers shall be paid out of the Construction and Maintenance Fund. The District will make progress payments monthly as the work proceeds, or at more frequent intervals as determined by the district engineer, on estimates approved by the district engineer. If requested by the district engineer, the contractor shall furnish a breakdown of the total contract price showing the amount included therein for each principal category of the work, in such detail as requested, to provide a basis for determining progress payments. In the preparation of estimates the district engineer, at his discretion, may authorize material delivered on the site and preparatory work done to be taken into consideration. Material delivered to the contractor at locations other than the site may also be taken into consideration (1) if such consideration is specifically authorized by the contract and (2) if the contractor furnishes satisfactory evidence that he has acquired title to such material and that it will be utilized on the work covered by this contract.

(c) In making such progress payments, there shall be retained 10 percent of the estimated amount until final completion and acceptance of the contract work. However, if the directors, at any time after 50 percent of the work has been completed, find that satisfactory progress is being made, they may authorize any of the remaining progress payments to be made in full. Also, whenever the work is substantially complete, the directors, if they consider the amount retained to be in excess of the amount adequate for the protection of the district, at their discretion, may release to the contractor all or a portion of such excess amount. Furthermore, on completion and acceptance of each separate project, public work, or other division of the contract, on which the price is stated separately in the contract, payment may be made therefor without retention of a percentage.