All contracts entered into, amended, or modified pursuant to authority contained in this chapter shall include a clause to the effect that the Comptroller General of the United States or any of his duly authorized representatives shall, until the expiration of three years after final payment, have access to and the right to examine any directly pertinent books, documents, papers, and records of the contractor or any of his subcontractors engaged in the performance of and involving transactions related to such contracts or subcontracts. Under regulations to be prescribed by the President, however, such clause may be omitted from contracts with foreign contractors or foreign subcontractors if the agency head determines, with the concurrence of the Comptroller General of the United States or his designee, that the omission will serve the best interests of the United States. However, the concurrence of the Comptroller General of the United States or his designee is not required for the omission of such clause—