Defendant Robert Martin gave his American Express credit card to E. L. McBride, a business associate, for use in a joint business venture. McBride returned the card to Martin and then disappeared. Martin claimed he had orally authorized McBride to charge up to $500 on the card. But McBride had actually charged approximately $5,300 to Martin's card. Martin also testified that he had previously sent a letter to plaintiff American Express asking them not to allow total charges or his account to exceed $1,000. Is Martin liable for $50 or $5,300?