The bonds issued under the provisions of this chapter, or such portion of such bonds as may remain unsold, if the same is ordered as hereinafter provided, may be issued to the contractor constructing the improvement in payment therefor, or the ordinance directing the issue of such bonds may provide that the same may be sold by some duly authorized officer or officers of the city in the manner prescribed therein, at not less than their par value and accrued interest, and that the proceeds thereof shall be applied in payment of the cost and expense of the improvement.