If there are outstanding bonds of the district at the time of the filing of the petition for exclusion of lands, the holders of such outstanding bonds may give their assent, in writing, to the effect that they severally consent that the lands mentioned in the petition, or such portion thereof as may be excluded from the district by order of the board, may be excluded from the district; and if such lands, or any portion thereof, are thereafter excluded from the district, the lands so excluded shall be released from the lien of such outstanding bonds.