Bava Metzia 75 - When Commodity Loans are Allowed
Although commodity loans are generally prohibited , a person may say to his fellow, "Lend me a bag of wheat until my son comes back to unlock my wheat, or until I find the key." It is deemed as if the lender acquires the other wheat right away, and consequently, if its value increases, it is as though the lender’s own wheat increases, and there is no question of interest.
Nevertheless, Hillel prohibits it. Hillel also wanted to prohibit a woman to lend a loaf to her friend, to be repaid with another loaf, unless they assess the value at the time of the loan, but the Sages did not agree with him.