Qitzur Shulchan Arukh – 65:16
It is prohibited to buy grain or another commodity by paying in advance to be given the grain at a later date, because we are concerned that meanwhile the grain will become more valuable [ie: will increase in price] afterward when he gives him the grain. It will come out that the buyer receivs more than the value of his money, by having paid in advance.
However, if he has to sell at this time, all the grain that he is putting up for sale, even though he will only deliver to the buyer at a later date, it is permissible. This is because whatever one has one can sell even at a very low price, if he wants to do so. Even if the grain is not yet fully ready but needs another one activity or two activities, it is considered as if it were ready [for sale] and may be sold. But if it still needs three more activities, [sale] is forbidden.
We’re now jumping back to continue the discussion of commodities speculation in 65:11. In sei’if 11, though, the commodity was borrowed now and the change in commodity value can effectively be interest. Here, it’s a sale. The dealing becomes an interest situation because ownership is only assumed at a later date.
If the person is hard-pressed for cash and the commodity is ready for sale or nearly so, then the deal can be framed as a undervalued sale of commodity for immediate ownership, without a time element offsetting the difference in price, and thus permissable.