No seller in a personal solicitation sale may offer to pay a commission or give a rebate or discount to the buyer in consideration of the buyer’s giving to the seller the names of prospective purchasers or otherwise aiding the seller in making a sale to another person, if the earning of the commission, rebate, or discount is contingent upon an event that is to happen subsequent to the time the buyer agrees to buy. Any sale made in respect to which a commission, rebate, or discount is offered in violation of this chapter is voidable at the option of the buyer.