Before the grapes ripen, a buyer can pay in advance for the wine yield of a vineyard. He pays much less than the estimated value of the wine.
Rav forbids it: since in the future the wine will be worth much more, it resembles "reward for waiting," or interest. Shmuel permits it: sometimes a misfortune can befall the vineyard, and the reduced price is on account of accepting risk. And Rav? – The small chance of loss does not justify the large price reduction. The law follows Rav. But Rav agrees to Shmuel where harvesting is done with the aid of oxen, because of the high risk to the oxen.