Whenever in the opinion of the board it appears that the total amount of any assessment previously levied and assessed and which has become a lien upon land in the drainage district, including assessments which have been bonded pursuant to the provisions of Part 5 of this division, is greater than required to meet all obligations incurred or to be incurred for the purposes for which the assessment was levied, the board may by resolution entered in its minutes, order the release of the lien of and abandonment of any part of the charges against any tract so assessed not required to be collected.