If a qualified purchaser under the terms of this subchapter desires to purchase a vessel to be constructed in accordance with an application for construction-differential subsidy under this subchapter, the Secretary of Transportation may, in lieu of contracting to pay the entire cost of the vessel under section
1152 of this Appendix, contract to pay only construction-differential subsidy and the cost of national defense features to the shipyard constructing such vessel. The construction-differential subsidy and payments for the cost of national defense features shall be based upon the lowest responsible domestic bid unless the vessel is constructed at a negotiated price as provided by section
1152
(a) of this Appendix or under a contract negotiated by the Secretary of Transportation as provided in section
1152
(b) of this Appendix in which event the construction-differential subsidy and payments for the cost of national defense features shall be based upon such negotiated price. No construction-differential subsidy, as provided in this section, shall be paid unless the said contract or contracts or other arrangements contain such provisions as are provided in this subchapter to protect the interests of the United States as the Secretary of Transportation deems necessary. Such vessel shall be documented under the laws of the United States as provided in section
1153 of this Appendix. The contract of sale, and the mortgage given to secure the payment of the unpaid balance of the purchase price, shall not restrict the lawful or proper use or operation of the vessel, except to the extent expressly required by law.