((a)) ** Presidential authority to enter into agreement; modification or continuance of existing duties** Whenever the President determines that any existing duties or other import restrictions of any foreign country or the United States are unduly burdening and restricting the foreign trade of the United States and that the purposes of this chapter will be promoted thereby, the President—
((1)) during the 5-year period beginning on , may enter into trade agreements with foreign countries or instrumentalities thereof; and
((2)) may proclaim such modification or continuance of any existing duty, such continuance of existing duty-free or excise treatment, or such additional duties, as he determines to be required or appropriate to carry out any such trade agreement.
((b)) ** Limitation on authority to decrease duty**
((1)) Except as provided in paragraph (2), no proclamation pursuant to subsection (a)(2) shall be made decreasing a rate of duty to a rate below 40 percent of the rate existing on .
((2)) Paragraph (1) shall not apply in the case of any article for which the rate of duty existing on , is not more than 5 percent ad valorem.
((c)) ** Limitation on authority to increase duty** No proclamation shall be made pursuant to subsection (a)(2) increasing any rate of duty to, or imposing a rate above, the higher of the following:
((1)) the rate which is 50 percent above the rate set forth in rate column numbered 2 of the Tariff Schedules of the United States as in effect on , or
((2)) the rate which is 20 percent ad valorem above the rate existing on .