A intended to resell an imported primary product. So he made an offer to B. B accepted the offer, but required A to provide a certificate of origin. Two weeks later, A received an L/C from B. And when A was just about to ship the goods according to the L/C, he was notified by the commodity inspection authority of his country that the certificate of origin could not be issued because the goods were not domestically made. A then requested B to cancel the clause in the L/C concerning the requirement of certificate of origin, but B refused. Disputes thus arose between A and B. A argued that he never agreed to the requirement of providing the certificate of origin and therefore he had no obligation to do so. B insisted that A did have this obligation. Give your views on the disputes in the light of the CISG.