Alan purchased shoes from Barbara on open account. Barbara sent Alan a bill for $10,000. Alan wrote back that two hundred pairs of the shoes were defective and offered to pay $6,000 and give Barbara his promissory note for $1,000. Barbara accepted the offer, and Alan sent his check for $6,000 and his note, in accordance with the agreement. Barbara cashed the check, collected on the note, and one month later sued Alan for $3,000. Is Barbara bound by her acceptance of the offer?