Gemara

Version #1 (Rav Yosef): Shmuel permits a seller to compensate the buyer for Shevach (improvements he made to the land) if the land will be taken from him if the seller has land (and will give it to pay for the Shevach).

Question (Abaye): If one has land, may he lend Se'ah b'Se'ah (a volume of wheat on condition to return the same volume? We know that this is forbidden, lest the price of wheat rise!)

Version #2 (Rav Yosef): Shmuel permits compensation for Shevach if the buyer acquired through Chalipin (to obligate the seller from the beginning.)

Question (Abaye): May one lend Se'ah b'Se'ah if he does Chalipin?! (end of Version #2)

Answer (Rav Yosef): Se'ah b'Se'ah is a loan. I discuss one who sold. (If he pays with land or did Chalipin, we are not concerned lest it look like Ribis.)

72b (Rav Huna): We may not borrow Se'ah for Se'ah even though there are Peros in the market.

Question: Rav Huna permitted Talmidim to borrow in Tishrei and repay Peros in Teves (at the cheaper price of Teves), because the money could be used now to buy Peros in Hini or Shili (other places)!

Answer: Originally, Rav Huna ruled that it was forbidden, until he heard that R. Shmuel bar Chiya permits it.

75a (Mishnah): One may not say 'lend to me a Kor of wheat on condition that I will return a Kor after the harvest.' He may ask for a loan until his son comes or until he finds the key (to get to wheat he has now);

(R. Chiya - Beraisa): One may not lend wine (or oil) when he does not have even a drop of wine (or oil);

Inference: If he has, he may borrow many drops.

Rishonim

Rif and Rosh (43a and 5:61): Rav Huna initially forbade to borrow Se'ah b'Se'ah (i.e. on condition to return a Se'ah, lest it be worth more when he returns it) even though there are Peros in the market. After he heard that R. Shmuel bar Chiya permits it, he also permitted. This is the Halachah. One may borrow and pay back Stam, based on the market price.

Nimukei Yosef (DH Gemara, Garsinan): A Mishnah permits Se'ah b'Se'ah when the borrower has some of what he borrows. He need not tell the lender; he borrows with this intent. The Ran explains that the lender immediately acquires what the borrower had; it rose in value in the lender's Reshus.

Question (Beis Yosef YD 162 DH v'Im Yesh): What does he teach? He does not permit only when he borrows with intent to repay from what he had initially, for one with a drop of wine may borrow much! He cannot mean that he must intend that the lender acquire drop for drop, for if so, why do we permit an ignoramus who does not know this?!

Answer (Bach DH v'Im): The Rambam holds that the Heter of a market price is only when both of them know this. The Nimukei Yosef teaches that the Heter of when the borrower has some of what he borrows is even if the lender does not know, since the borrower intends for this Heter.

Taz (3): The Rambam requires that both know the market price. When the borrower has, the lender need not know. The Bach holds that also then, both of them must know. This is wrong. If even the borrower did not know, the Nimukei Yosef forbids, but the Ran permits, for Chachamim acquired it to the lender.

Note: The Bach said 'if not, one of them intended for Isur.' The Taz understood that also the lender must know. Perhaps the Bach means that 'if not (that the borrower knows that he has), one of them (the borrower) intended for Isur.' Perhaps we are not concerned for the lender, for he assumes that the borrower would not borrow unless he had some (for otherwise, it is forbidden)!

Shach (7): Perhaps the Beis Yosef means like the Bach says. Even the Bach says so only l'Chatchilah. If one lent without intent for this, the Bach agrees that he may and must pay (Se'ah b'Se'ah). The Rosh and Tur connote like this.

Rif (45a): The Halachah is, if one has a Se'ah, he may borrow many Korim. Chachamim permit to lend Stam (without fixing a price) and repay Stam. This is when there is a market price, like Rav Huna taught, or when the borrower has some of that species, like R. Yitzchak. In such a case, it is like a loan until his son comes or until he finds the key.

Rambam (Hilchos Malveh 10:1): One may contract (to supply Peros) when there is a fixed market price, also this permits lending Se'ah b'Se'ah Stam without fixing a time. E.g. if there was a fixed market price known to both of them, and he lent 10 Sa'im, he must pay back 10 Sa'im even if the price increased. When he borrowed, he could have bought and returned to him, for no time was fixed.

Toras Chayim (Bava Metzia 72b DH v'Nir'eh): If he fixed a time, and if the borrower will not pay by then he will collect like the cheap price then, this is Agar Natar (compensation for delaying payment, i.e. Ribis).

Rosh (61): We said above that one may lend on (condition to be paid with) Peros only if he has, even if there is a market price. There is different. Since he lent money, it looks like Ribis. Chachamim permit to lend a Se'ah based on the market price, like they permitted 'until my son comes or until I find the key.' Even if the borrower has no money, he could buy Peros on credit, since there is a price, or borrow (money) from another. The Ra'avad says that the Heter to lend Se'ah b'Se'ah based on a market price is only when the borrower has money and could buy, so it is as if he has Peros. We permit because he could buy elsewhere. If he has no money, he could not!

Rosh (75): It is permitted only if the borrower has. Rav Huna discusses when there is no market price. R. Yitzchak permits borrowing any amount at once even if he has only a little. When there is a market price, even one without anything may borrow. The Halachah follows Chachamim, who say that one may borrow Stam and pay back Stam.

Hagahos Ashri: Rashi permits even if there is no fixed price. The Rif requires a fixed price.

Beis Yosef (YD 162 DH a'Ha): Tosfos (72b DH Ein) asks why we need a market price. If suffices that he has money and can buy Peros! Some texts do not mention a fixed price, only that there are Peros in the market. The text of Rashi, Rif, Rambam and Rosh mentions a fixed price; one may not rely on Tosfos.

Teshuvas Rosh (108:7,15): If Reuven borrowed something and claims that he had none of that species, and Shimon (the lender) says that he had, Shimon is not believed. Reuven can say 'I cannot pay the higher value now, for this is Ribis.' Shimon cannot be Makneh to Reuven through someone else, for this harms Shimon. We do not say that surely Shimon borrowed in a permitted way, for there is no Vadai Isur; he did not expect the commodity to rise in price. If it will rise, he can pay its initial value without any Isur. Shimon must swear that he did not have, and that there was no market price.

Note: There is an Isur at the time of the loan to borrow Se'ah b'Se'ah! Perhaps people think that it is forbidden only if he will pay Ribis later.

Poskim

Shulchan Aruch (YD 162:3): If there was a fixed market price known to both of them, one may lend Se'ah b'Se'ah.

Beis Yosef (DH v'Chen Im): The Rambam requires that both of them know the price. If not, the other intends that the loan be paid after the price rises, which is Ribis. However, perhaps it suffices that they know that there is a fixed price. Since the borrower could buy and return, they do not intend for Ribis. Teshuvas ha'Rosh does not require them to know of a market price. He says that it suffices to clarify that there was a market price.

Gidulei Terumah (46:5:1): Why didn't the Tur mention that the Rosh argues with the Rambam? He must hold that they do not argue. The Rambam teaches that one may borrow l'Chatchilah when both know. The Rosh teaches that if there was a market price, the borrower pays pack a Se'ah, even if they did not know that there was a market price. Alternatively, he teaches that if the lender can prove that there was a market price, the borrowed must prove that one of them did not know, for it is common knowledge. Surely, one would have told the other to avoid the Isur!

Beis Yosef (DH Im): When there is a market price, we permit, because the money could be used now to buy Peros in Hini or Shili. This shows that a market price in one place permits other nearby places. Also the Mordechai and Yerushalmi say so.

Beis Yosef (DH v'Hai): Some say that one may not contract based on the price in cities, because it is not fixed. They hold the same about Se'ah for Se'ah. This is why the Rambam said that the price was fixed.

Beis Yosef (DH veha'Ra'avad): The Ra'avad and Tosfos (72b DH Ein) say that we may rely on a market price only if the borrower has money. Piskei Rosh connotes like this, but in a Teshuvah he did not require this. Sefer ha'Terumos (5:46:5) says that the Ra'avad requires that he has money. If he has something else that he could sell, perhaps the fixed price will change before he is able to buy. The Rashbatz says that the Rif and Rambam permit in every case. Also the Ramban says that the Ge'onim are unlike the Ra'avad; we rule like them.