An owner of a parcel of vacant land and a prospective buyer enter into a written contract. For a nonrefundable fee, the owner agrees to keep the prospective buyer's offer on a property open--for a mutually-acceptable period of time. If the prospective buyer decides to buy the vacant land, both parties have agreed to a purchase price. If the prospective buyer decides against purchasing the property, the owner will keep the nonrefundable fee received from the prospective buyer at the time the contract was signed. Which term most accurately describes the contractual agreement between these two parties?