Iowa Code § 311.29: Sale of certificates.

Upon the signing of each of the certificates by the chairperson of the board, the certificates shall be delivered to the county treasurer, who shall countersign them and who shall be responsible for them on the treasurer’s bond. The treasurer may apply the certificates in payment of warrants duly authorized and issued for improving the roads within the district, or the treasurer may sell the certificates for the best attainable price and for not less than par, plus accrued interest. The certificates shall be retired in the order of their numbering.