If the actual amount to be paid by the distribution district to the United States or to the district under the contract to be executed is substantially less than the estimated amount, the assessment may be reduced proportionately on each parcel by recomputing it, based on the actual amount to be repaid as set forth in said contract. The reapportionment or a statement that the assessment on each parcel has been reduced by a designated percentage shall be recorded in the same manner as the order levying the original assessment.