Qitzur Shulchan Arukh – 65:11
It is prohibited to lend someone 100 se’ah of grain[on the grounds] that he return 100 se’ah of grain, even one kind for its own kind, because maybe grain will increase in value in the interrum and it will turn out that he will return more than he borrowed. Rather, he should make [the deal] on money, that if the grain goes up in value, he would only return the monetary value.
If the borrower only has a little of this kind, he can borrow even a number of korin [1 kor = 20 se’ah].
Similarly, if some type of grain has a fixed market price, it is permissible to lend it out even if the borrower hasn’t any of that kind of grain.
All this [is speaking of] one kind [of grain being repaid] in its own kind. However, where one kind [is being repaid] in a different kind, like borrowing a se’ah of wheat in [in exchange for a return of] a se’ah of millet, it is forbidden in all cases. Even if they both have the same market price and [even if] he does have millet on hand.
With a small thing, where it is not the norm to care about price changes, it is allowed in all cases. Therefore, a woman may borrow a loaf of bread from her friend [to return a loaf of bread].
It is assur to speculate in commodities where the counterparty is also a Jew. (On a pragmatic level, the presence of an exchange so that you do not have a particular counterparty or a majority of Jewish inverstors alleviates the problem.) The change in commodity value would be prohibited interest. This prohibition is rabbinic, and therefore if the question is asked after it is collected, it need not be repaid to the borrower.
I don’t know why the borrower is allowed to borrow more of a commodity he already has on hand to repay in that commodity. It’s the opinion of R’ Yitzvchaq on Bava Metzia 75a. For some reason the legislation doesn’t include this case. If you have a motivation for why, kindly share it.
Where the item is not a commodity with a set market price, for example I’m borrowing your snow blower to return that same particular snow blower, there is no problem. Where the item’s change in value isn’t large enough to be a futures trade, there is also no problem — the difference in value is ignorable.
Interesting for the purpose of this series on the Qitzur is how far we are expected to distance ourselves from collecting interest. Here there is a derabbanan against lending a commodity because it might go up in value enough to qualify as interest.