When he determines it to be in the national interest, the President is authorized to make voluntary contributions on a grant basis to international organizations and to programs administered by such organizations, and in the case of the Indus Basin Development Fund administered by the International Bank for Reconstruction and Development to make grants and loans payable as to principal and interest in United States dollars and subject to the provisions of section
2151t
(b) of this title, on such terms and conditions as he may determine, in order to further the purposes of subchapter I of this chapter.