When a district has levied an assessment and it appears to the board that it is unnecessary to collect all, or some portion of it, and it also appears to the board that the assessment or some part thereof is greater than required to meet all obligations incurred or to be incurred for the purposes for which the assessment or part thereof was levied, the board may, by resolution, request the board of supervisors to cancel all or any portion of the assessment including any interest thereon.