Iowa Code § 427.18: Token tax liability accrues.

If property which may be exempt from taxation is acquired after July 1 by a person or the state or any of its political subdivisions, the exemption shall not be allowed for that fiscal year and the person or the state or any of its political subdivisions shall pay the property taxes levied against the property for that fiscal year, and payable in the following fiscal year. However, the seller and the purchaser may designate, by written agreement, the party responsible for payment of the property taxes due.