All moneys accruing, from time to time, from taxes levied prior to the annexation, and all assessments against private property for public improvements, for which the contractors shall have been paid, shall be applied to the indebtedness of the territory annexed; and it shall be lawful for the councils and mayor of the enlarged city to fund the debt of any municipality or school district so annexed, and, for so doing, to issue the bonds of the enlarged city, in accordance with existing laws applicable to said city, and to provide for an annual tax, upon the subjects of taxation within the limits of the annexed territory, sufficient to retire said bonds as they mature.