Iowa Code § 420.224

Property which may be sold at tax sale to any such city shall not be offered at any sale for taxes or special assessments, collectible by such city, while it holds the certificate of purchase thereof or tax deed thereon except that if any special assessment or installment thereof levied by any such city prior to April 22, 1941, shall be or become delinquent after purchase of such property at tax sale by the city, then the property against which the same was levied may be sold therefor only at the first regular tax sale of such city occurring within such a period of time after delinquency that sale for such assessment or installment might lawfully be made at such first regular tax sale. Nothing in sections 420.220 to 420.229 shall prevent the sale of property for any unpaid taxes collectible by the county.