Not totally sure I understand the question. Are you currently under contract due to a listing agreement on a property and looking to rescind the contract? What are your reasons for wanting to cancel your listing if this is the case? Or are you looking to rescind the listing agreement while the house is under contract with a buyer?
Iowa does have what are called non-statuatory liens in which a real estate company could ask for a judgement against you that would have to be paid before a home could be sold if the judge felt the real esate company had earned and should be paid a commission. Most likely, if they had brought you an offer and you had accepted the offer, but later decided you didn't want to sell the home. I am not sure, without having all the details, if that would even be a consideration here.