cellio | daf bit: Bava Metzia 72

The mishna teaches: a man must not fix a price for produce until the market price is known; once the market price is established, a seller and buyer may agree on a fixed price. A buyer may further agree on a price for certain goods that the seller does not yet have in hand: stalks of grain not yet processed into wheat, grapes not yet turned into wine, olives not yet pressed into oil, clay not yet turned into earthenware, and lime in the kiln. In all of these cases the seller is treated as if he is in possession of the finished product (e.g. the wine from the grapes) even though manufacture is not complete and delivery will occur later. Normally we would be concerned about usury, should the market price change between the sale and the delivery, so in these cases we are told that it is nevertheless permitted and we do not count gains as earning interest. (72b)

Noteworthy to me is that these are (almost) all commodities. It doesn't matter which local farmer you get your wheat from, for instance; it's all the same. (Modern analogy: gas stations.). The "almost" is because earthenware can vary significantly based on the skill and artistry of the potter, but maybe we're talking here about basic, utilitarian products.

Today's daf is 73.