Any contracts or agreements approved prior to June 26, 1936, by the Secretary of the Interior between the authorities of any tribe, band, or group of Indians and their attorneys for the prosecution of claims against the United States, which provide that such contracts or agreements shall run for a period of years therein specified, and as long thereafter as may be required to complete the business therein provided for, or words of like import, or which provide that compensation for services rendered shall be on a quantum-meruit basis not to exceed a specified percentage, shall be deemed a sufficient compliance with section 81 of this title: Provided, however, That nothing herein contained shall limit the power of the Secretary of the Interior, after due notice and hearing and for proper cause shown, to cancel any such contract or agreement: Provided further, That the provisions of this section and section 81b of this title shall not be construed to revive any contract which has been terminated by lapse of time, operation of law, or by acts of the parties thereto.