We will eat this cooked food of you. You may cook these raw things (that we give in return). In such an agreement there is no sin.
वयं सिद्धमशिष्यामो भवान् साधयतामिदम्। एवं संवीक्ष्य नियमेन्नाधर्मोऽस्ति कथंचन॥