If assessments are made on the basis of estimated cost, and upon completion of the improvement the cost is found to be less than the estimated cost, the excess amount shall be entered on the lien docket as a credit on the appropriate assessment and shall be apportioned and credited to the owners of the assessed property on the same basis as the original assessment. In the event that any assessment has been paid in full, the owner of the assessed property at the time the council declares the excess shall be paid a cash refund of their proportionate share of the excess amount.