The city manager shall within sixty days after the completion of each special assessment project compile the actual cost thereof and certify same to the council. Should the assessment prove larger than necessary by five per cent or less, the council may place the exCess in the city treasury; if more than five per cent, the excess shall be refunded pro rata according to assessments. In either case, the council 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.