Gemara

63b (Rabah and Rav Yosef): One may contract to supply Peros even if he has no Peros, because he does not give anything extra to the buyer. Had the buyer kept his money, he could have bought Peros cheaply elsewhere.

Question (Abaye): If so, a loan of a Se'ah on condition to return a Se'ah should be permitted, for the borrower does not give anything extra to the lender. It would not have spoiled!

Answer (Rav Yosef): A loan is forbidden. We permit only a sale.

72b: 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)!

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);

Hillel forbids this.

(R. Yitzchak): if one has a Se'ah, he may borrow many Sa'im.

Support (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.

(Mishnah): Similarly, Hillel forbids a woman to lend a loaf to her neighbor until she sets a price for it, lest wheat rise in price and she will give (back a loaf worth more, which is) Ribis.

(Rav Yehudah): Chachamim permit Stam.

(Rav Yehudah): If people lend to each other and are insistent to get back no less than they gave, according to Hillel, they transgress Ribis.

Rishonim

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. Hillel forbids lending a loaf until setting a price. Chachamim permit as long as there is a market price.

Rebuttal (Ba'al ha'Ma'or): Chachamim permit a loaf even without a market price! Its value does not change much. If the borrower does not have a loaf, others do. This is like manure; one may contract for it at any time of the year.

Milchamos Hash-m: The Ba'al ha'Ma'or misunderstood the Rif! Rav Yehudah said 'we may borrow and pay Stam.' He refers to Se'ah b'Se'ah, not to a loaf. Bahag explained that this is even without a market price. The Rif teaches unlike Bahag; it is only if there is a market price. This is why it says 'wemay borrow', for it does not refer to the case of a woman borrowing a loaf). The Rif agrees that a loaf is permitted without a fixed price!

Gra (YD 162:5): R. Chananel (cited in Milchamos Hash-m) connotes the way Hagahos Ashri and Beis Yosef understand the Rif (i.e. like the Ba'al ha'Ma'or did).

Rambam (Hilchos Malveh 10:2): If one wants to borrow something and has none of that species, and there is no market price, or they did not know the market price, he may not borrow Se'ah b'Se'ah until fixing a price. If he borrowed without fixing a price, and the Peros declined in value, he pays back the measure or weight he borrowed. If the price increased, he pays back what it was worth at the time of the loan. Even if one has some of the species he wants to borrow, or there was a market price, one may not borrow Peros for Peros for a fixed time. Rather, he borrows Stam and pays back at any time.

Rebuttal (Ra'avad): We never heard such a Chidush! Rather, if he has, or there is a market price, one may borrow Stam without fixing a price, even for a fixed time.

Magid Mishneh: The Rambam explains that the Reisha forbids lending for a fixed time. This is even if he has, similar to the Seifa (until my son comes). Other Meforshim explain that whenever he has, it is permitted even if he fixed a time. If so, the Mishnah should have said that it depends only on this! It seems that one fixes a time because he expects the price to rise, and there is a Chazakah that the borrower will not pay early. The Rashba disagrees. Fixing a time benefits the borrower; the lender cannot force him to pay before the time. It is unusual to pay early, but no one forbids doing so.

Rambam (3): One may not say 'lend to me a Kor of wheat, and I will return a Kor after the harvest'. He may ask for a loan until his son comes or until he finds the key.

Rosh (75): The Mishnah forbids a loan 'until after the harvest', for typically that is when he will have wheat to pay back. The same applies if no time was fixed. It is permitted only if the borrower has, or if there is a market price. The Halachah follows Chachamim, who permit to borrow Stam and pay back Stam.

Poskim

Shulchan Aruch (YD 162:1): One may not lend Se'ah b'Se'ah.

Shach (1): Since the Isur is mid'Rabanan, if one transgressed, Beis Din does not force him to return the Ribis returned.

Taz (1): This is permitted in the way of a sale. I.e. one sold a Se'ah, and at a fixed time pays with a different Se'ah in place of it. He is prone to gain or lose. Even regarding a loan, this is forbidden only mid'Rabanan. Therefore, it is totally permitted regarding a sale. The Torah permits in the way of a sale even if he is more prone to gain than to lose.

Shulchan Aruch (ibid): This is even if he did not fix a payment date.

Beis Yosef (DH Bein): Even though the Mishnah forbids a loan 'until after the harvest', this is merely a typical case. Alternatively, it is for parallel structure with the Seifa, which permits 'until my son comes or until I find the key.'

Shulchan Aruch (ibid): The same applies to all commodities. We are concerned lest the commodity increase in price, and he pays back more than was lent. It is permitted only if a price is fixed, and if the commodity will rise in value, he will pay back the price fixed.

Pischei Teshuvah (3, citing Beis Efrayim YD 43): If the price declined, he must pay back wheat. He may pay money only if the price rose.

Shulchan Aruch (ibid): If one borrowed without fixing a price and the commodity rose in value, he pays back what it was worth at the time of the loan. If the price decreased, he pays back a Se'ah, the amount he borrowed.

Rema: Some permit lending a loaf for a loaf. It is a small amount, so people are not particular about it with each other. The custom is to be lenient.

Beis Yosef (DH Ishah): The Tur, Rashi and many others hold that we hold like Chachamim who argue with Hillel regarding both laws. Chachamim were not stringent about a small amount. The Rif requires a market price even for a loaf. It seems that the Rambam agrees, for he did not distinguish. They must explain that Hillel mentioned a loaf not because Chachamim are more lenient about it, rather, to teach that he is stringent, even though it is a small amount. Rav Yehudah taught that people who lend to each other transgress Ribis according to Hillel. This is when the borrower has, or there is a market price, for if not, even Chachamim forbid. The Rosh did not say whether Chachamim permit only when there is a market price. Perhaps he relies on Rashi. The Tuf's Kitzur Piskei Rosh permits lending a loaf for a loaf (in every case).

Shulchan Aruch (YD 162:2): If one has a small amount of a species, he may borrow many Sa'im and repay Se'ah b'Se'ah.

Rema: This is when the borrower can pay back whenever he wants. If the lender stipulated not to accept payment until it is expensive, it is forbidden.

Beis Yosef (YD 162 DH Kosav ha'Rambam): R. Yerucham says that the Rif holds like the Rambam (who always forbids when a time was fixed). I see no source to say so. Teshuvas ha'Rosh (108:15) is like the Ra'avad. However, if he stipulated not to pay until a fixed time, one may not rely on a market price, but if he has the same species, it is permitted. Semak and Kol Bo forbid if he stipulated not to pay until the price rises.

Shach (11): Why did the Rema say 'if he stipulated not to accept payment until it is expensive'? The Rosh is stringent whenever he stipulated not to accept payment before a fixed time! Perhaps he merely used the Semak's wording. Alternatively, he teaches that if the price rose, he does not pay the higher price. I say that if he fixed a time lest the price drop, all forbid even if he did not stipulate not to accept payment before the time. Poskim argue with the Rambam only because fixing a time helps the borrower. When it helps the lender, he can refuse to accept early payment!