As we have seen on the previous dapim, when offering a discount on a sale for pre-payment, there may be a problem of ribit (forbidden interest), since the pre-payment can be viewed as a loan, and by offering a discount, the merchant may be seen as repaying more than he received.
The Mishna on today’s daf teaches that there is a difference between offering a discount on rent and offering a discount on a purchase. At the time that someone rents his yard he is allowed to tell the renter that he has a choice – he can choose to pay ten sela’im for the entire year if he pays up-front (a total cost of ten sela’im), or else he can pay one sela every month (a total cost of twelve sela’im). If he was offering his field for sale, however, he would not be allowed to give the purchaser a choice of either paying 1,000 zuz up-front or 12 maneh (1,200 zuz) at harvest time, since that would be considered ribit.
The Gemara explains the difference between the two cases by quoting Rabba and Rav Yosef who agree that rent is only owed at the end of the agreed upon period. Therefore, when offering a choice of paying ten sela’im now or twelve sela’im if payment is made at the end of the year, the true value of the rental is twelve sela’im, and the owner is simply offering a discount for early payment. In situations of buying and selling, however, the purchase price should be paid at the time that the sale is made. Therefore the perspective taken by the Sages is that the cost for “early” payment is the base price and paying more for a “late” payment is effectively ribit on a loan.
When this ruling appears codified in the Shulhan Arukh (Yoreh De’a 176:6) the Rema points out that even in cases of renting we can find situations that ribit can be a problem. If the owner and the renter agreed on a final price for the year, and then the owner says that if the renter wants to spread the payments out every month he expects to receive more, that would be considered a case of forbidden interest.