The President shall not be required to apply or maintain any Presidential action under this part—
(1) in the case of procurement of defense articles or defense services—
under existing contracts or subcontracts, including the exercise of options for production quantities, to satisfy requirements essential to the national security of the United States;
if the President determines in writing and so reports to Congress that the person or other entity to which the Presidential action would otherwise be applied is a sole source supplier of the defense articles or services, that the defense articles or services are essential, and that alternative sources are not readily or reasonably available; or
to products or services provided under contracts entered into before the date on which the President publishes his intention to take the Presidential action.