Iowa Code § 384.5: Excess tax.

A tax levied for the debt service fund is not invalid if it raises moneys in excess of those needed for a specific purpose. Only excess moneys remaining after retirement of all indebtedness payable from the fund may be transferred from the debt service fund to any other city fund, subject to the terms of the original bond issue, and as provided in rules promulgated by the city finance committee created in section 384.13.