Whenever any city of the first class has increased or has authorized the increase of its indebtedness without the assent of the electors of such city, or shall hereafter so increase or authorize the increase of its indebtedness, and the purpose of such increase or authorized increase has proved or shall prove to be impracticable, impossible or inadvisable, the council of such city may by their ordinance, which shall require the affirmative vote of two-thirds of all of the members of the council, so declare, and (a) may provide for the use of the money so borrowed or authorized to be borrowed for any other municipal purpose for which such indebtedness could have originally been lawfully incurred and upon the certificate of the City Controller as required by law for the original incurring of such debt, or (b) may rescind in whole or in part the authority to borrow such money. The council of such city may rescind in like manner in whole or in part the authority to borrow such money if originally authorized with the assent of the electors.