The rule is well known, שוה כסף ככסף. He has Chametz in your domain and says you should keep it. In Bava Kama 46b we find the Gemara quoting the saying that if someone owes you money take whatever he offers for soon even that won’t be available.
As the Rishonim there point out, usually we don’t allow payment in such form since you received money and should therefore be courteous enough to return money. In this case however, obviously accepting the Chametz as value is part of the deal.