When any special assessment roll shall prove insufficient to meet the costs of the improvement for which it was made, the Commission may make an additional pro rata assessment, but the total amount assessed shall not exceed the value of benefits received by any lot or parcel of land. Should the assessment prove larger than necessary to five (5) per cent or less, the Commission may place the excess in the city treasury. If more than five (5) per cent, the excess shall be refunded pro rata according to assessments. In either case, the Commission may provide by resolution that the amount of any such excess may be allowed as a credit on the last installment where such installment still remains unpaid at the time the final cost of the improvement is determined.