If the Authority intends to pledge any future federal aid from any source to support repayment of any debt instrument issued under this subtitle: The aggregate principal amount of debt issued under this subtitle or Title 3, Subtitle 6 of this article that is secured by a pledge of future federal aid may not exceed $750,000,000; The date of maturity may not be later than 12 years after the date of issue; Notwithstanding § 3-215(d) of this article, if future federal aid is insufficient to pay the principal of and interest on the bonds issued under this subtitle when due, the tax levied under § 3-215 of this article, to the extent the proceeds of such tax are not necessary to provide the sinking fund required under § 3-215(c) of this article, is irrevocably pledged to the payment of the principal of and interest on the bonds issued under this subtitle as they become due and payable; The lien of the pledge under item (3) of this subsection shall at all times be subordinate to the lien of the pledge of such tax under § 3-215(d) of this article to the payment of principal of and interest on consolidated transportation bonds; and No part of the tax levied under § 3-215 of this article may be repealed, diminished, or applied to any other purpose until: The bonds issued under this subtitle and interest on them have become due and fully paid; or Adequate and complete provision for payment of the principal and interest has been made.