Bava Metzia 64 - Forward Contract on Gourds
Rav Kahana was once late to Rav's lecture. He heard that Rav kept mentioning the word "gourds," and he later asked the other students, what was Rav saying. They told him this: If one gave money to a gardener for large gourds, while the gardener currently only had small gourds, such a forward contract is allowed, because small gourds will grow into large ones - provided that the gardener supplies these same gourds.
One who makes a loan to his fellow may not dwell in a borrower's house for free, nor may he rent it from him for less than the usual price - because this constitutes interest.