Iowa Code § 452A.22: Tax collected on exempt fuel.

If an amount of tax represented by a licensee to a purchaser as constituting tax due is computed upon gallonage that is not taxable or the amount represented is in excess of the actual amount of tax due and the amount represented is actually paid by the purchaser to the licensee, the excess amount of tax paid shall be returned to the purchaser by the licensee. If the licensee fails to return the excess tax paid to the purchaser, the amount which the purchaser has paid to the licensee shall be remitted by the licensee to the department.