(a) Ten per centum shall be retained from each progress payment or estimate until the contract work is fifty per centum completed, after which no further moneys shall be retained from any progress payments or estimates paid thereafter, and when the entire contract work has been completed and accepted, the corporation shall, pending the payment of the final estimate, pay not to exceed fifty per centum of the amount of the retained percentage.

(b) Whenever in the judgment of the corporation the withholding of the retained percentage on account of the closing of the working season would be an injustice to the contractor, the corporation may, provided the district engineer certifies that the essential items in the contract have been completed in accordance with the terms of the contract and the provisions of this chapter, direct the district engineer to include in the final account such uncompleted items and pay therefor at the item prices in the contract upon the contractor depositing with the corporation securities equal to double the value of such uncompleted work. The deposit may be used by the corporation to complete the uncompleted portion of the contract and shall be returned to the contractor if it completes the uncompleted portion within a specified number of working days after it has been notified to proceed with the work.

(c) No certificates approving or authorizing a partial or final payment shall be made by the corporation until it is satisfied that all laborers employed on the work have been paid for their services for the last payroll period preceding the said partial or final payment. The corporation may, if it deems necessary, require an affidavit to such effect from the contractor or it may depend on any other source which it deems proper for such information.

(d) Contracts in force at the date of the enactment of this subdivision may, in the discretion of the corporation, be amended to provide for the withholding and the payments contemplated by the provisions of paragraph (a) of this subdivision, if the surety or sureties upon the performance and labor and material bonds given by a contractor upon any such contract shall consent in writing thereto.

(e) No such certificate authorizing or approving the first partial payment or any final payment to a foreign contractor shall be made unless such contractor shall furnish satisfactory proof that all taxes due the commissioner of taxation and finance by such contractor under the provisions of or pursuant to a law enacted pursuant to the authority of article nine, nine-A, twelve-A, twenty-one, twenty-two, twenty-eight, twenty-nine or thirty of the tax law have been paid. The certificate of the commissioner of taxation and finance to the effect that all such taxes have been paid shall be, for purpose of this paragraph, conclusive proof of the payment of such taxes. The term "foreign contractor" as used in this subdivision means, in the case of an individual, a person who is not a resident of this state, in the case of a partnership, one having one or more partners not a resident of this state, and in the case of a corporation, one not organized under the laws of this state. 8. Contingencies and extra work. Whenever the corporation determines that from any unforeseen cause the terms of any contract should be altered to provide for contingencies or extra work, it may, if funds are available for payment of the cost thereof, issue an order on contract therefor to the contractor, a copy of which shall be filed with the director of the budget and the state comptroller. The estimated expenditure pursuant to the order on contract shall not increase the total amount of the primary contract until the estimated expenditure shall have been approved by the corporation and a duplicate of such approval shall have been filed with the comptroller. No such extra work shall be commenced or undertaken until the corporation has issued an order on contract as herein provided. When such order on contract provides for similar items of work or materials which increase or decrease the itemized quantity provided for in the primary contract, the price to be paid therefor shall not exceed the unit bid price in the primary contract for such items. Agreed prices for new items of work or materials may be incorporated in the order on contract as the corporation may deem them to be just and fair and beneficial to the state, including the corporation. Whenever the corporation also determines that in the cases herein provided it is impracticable for it to ascertain in advance the just and fair prices to be paid by the state for new items of work or materials, the order on contract therefor may provide for performance of the work and the furnishing of the materials and equipment, in which event the contractor shall keep and shall make available at all times to the corporation such accounting records, data and procedure as may be required by the corporation.