If there are outstanding bonds of the district at the time of the filing of the exclusion petition, the holders of the outstanding bonds may give their assent, in writing, to the effect that they severally consent that the land 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. If the land or any portion thereof is thereafter excluded from the district, the land so excluded is released from the lien of such outstanding bonds.