Assessments shall be made by the board of assessment commissioners prior to the acquisition of an improvement or commencement of the construction of an improvement described in the ordinance organizing the central business improvement district or upon the completion of the improvements, at the discretion of the board of assessment commissioners. In the event, however, that such assessments prove ultimately to be in excess of the total cost, the surplus of assessments may be rebated to the owners of the property assessed. The amount to be rebated to each property owner shall be the proportion of the total rebate that the amount of the property owner’s assessment bears to the total amount of all assessments originally levied. The rebate may be applied as a prepayment of any unpaid amount of the assessment and any balance shall be paid to the owner at the time the rebate is made of the property assessed. In the event that the totality of the assessment ultimately proves to be less than the total cost of all the improvements in the central business improvement district, the municipality shall be authorized to increase the assessment on each property by an amount equal to the percentage that the original assessment on each property bears to the total assessment, multiplied by the total additional assessment required of the district.