If any of the outstanding bonds are payable from assessments upon all real property in the district, any real property excluded from the district by virtue of the exclusion of lands from said district shall, notwithstanding such exclusion, be considered a part of such district for the purpose of discharging such outstanding bonded indebtedness and shall be and remain liable to be taxed to pay the principal of and interest on such outstanding bonded indebtedness until such outstanding bonded indebtedness and the interest thereon is fully paid; except that the provisions of this section shall not apply to any outstanding bonds, the holders of which have assented to the exclusion of such lands from the district.