Qitzur Shulchan Arukh – 65:17
If the there is a fixed market price for grain, he may offer to buy for advance payment at that price, even though the seller has nothing [in his possession yet to sell]. Because, even if the grain becomes more expensive afterward, the buyer does not profit by paying in advance since he could have bought grain[from a 3rd party] at this time at the [same] market price.
Since the sale was permissible, even if afterward the price went up of the grain at the due date, and he does not want to give him the grain at the agreed price, he can decide on other merchandise to give him, or give him money of equal value to the present market value.
This differs from the loan of a commodity because the buyer could have bought the same grain up front and made similar profit. It would only be if next quarter’s wheat were being sold today at a different fixed market price than wheat itself (such as at the CME) is that our early discussion becomes relevant.
The second paragraph discusses a distinction the commodities future trader would call physical settlement (actually receiving the commodity) vs. cash settlement (receiving its value). In our case, cash settlement is allowed, as well as anything else of that value.