It is a misdemeanor for any person that is subject to this chapter to willfully refuse to pay for any farm product which such person purchases within 20 days after the date of the delivery of such product by the producer if he has the ability to pay and demand for payment is made, except in the case of a bona fide dispute, unless the time of payment is otherwise agreed upon in writing, or the payment of the purchase price is secured other than by a lien.