In order to clarify the original intent of Congress, hereafter, section 10a of this title and that part of section 10b(a) of this title preceding the words ''Provided, however,'' shall be regarded as requiring the purchase, for public use within the United States, of articles, materials, or supplies manufactured in the United States in sufficient and reasonably available commercial quantities and of a satisfactory quality, unless the head of the department or independent establishment concerned shall determine their purchase to be inconsistent with the public interest or their cost to be unreasonable