"(m) When essential to the proper administration of its corporate affairs or necessary for the proper transaction of its business or to carry indebtedness and issue bonds, it was organized, to construct indebtedness and issue bonds, subject to the approval of the Secretary of Finance. The bonded indebtedness of the District, of all classes, shall not at any time exceed thirty million pesos, and the issue thereof shall be subject to the conditions set forth in section five of this Act, as amended, and shall be limited to revenue-producing projects: Provided, That part of the amount borrowed shall be spent for sinking underground the water main pipes in Manila."