After September 30, 1997, financial assistance for rental housing assistance under the United States Housing Act of 1937 [42 U.S.C. 1437 et seq.] may not be provided to any Indian housing authority or tribally designated housing entity, unless such assistance is provided pursuant to a contract for such assistance entered into by the Secretary and the Indian housing authority before such date. Any such assistance provided pursuant to such a contract shall be governed by the provisions of the United States Housing Act of 1937 (as in effect before the date of the effectiveness of this chapter) and the provisions of such contract.