If assessments have been made on the basis of estimated cost and upon completion of the improvement project the cost is found to be less than the estimated cost, the council shall ascertain and declare the same by resolution, and when so declared, the excess amounts shall be entered on the City lien record as a credit upon the appropriate assessment. Thereafter, the person who paid the original assessment, or that person’s legal representative or successor, shall be entitled to repayment of the excess amount. If the property owner has filed an application to pay the assessment by installment, the owner shall be entitled to such refund only when such installments, together with interest thereon, are fully paid. If the property owner has neither paid such assessment nor filed an application to pay in installments, the amount of the refund shall be deducted from such assessment, and the remainder shall remain a lien on the property until legally satisfied.