Gemara

(Beraisa): If Reuven has houses, fields and vineyards (worth 200 Zuz), but cannot sell them, he may take Ma'aser Oni, up to half (100 Zuz).

Question (Rabah): What is the case?

If the value of everyone's land declined, Reuven may receive even more! (He is like any Oni, who may receive a gift of any size.)

Answer #1: Other people's land did not decline. Reuven's land declined, because people know that he must sell.

Rejection: If so, he may not receive any gifts for the poor!

Answer #2 (Rabah): Rather, the case is (in Reuven's area), land is worth more in Nisan, and less in Tishrei. Everyone else waits until Nisan to sell. Reuven cannot wait, and he must sell now. The seasonal cheapening of land is never worse than half the value. (Land worth 200 in Nisan is worth at least 100 in Tishrei.)

Rishonim

Rif: If Reuven has houses... but cannot sell them, he may take Ma'aser Oni, up to half. If the value of everyone's land declined (and Reuven can still sell for 200), he may not take at all! If only his land declined, because people know that he must sell, he may receive even more! Rather, land is worth more in Nisan, and less in Tishrei. Everyone else waits until Nisan to sell. Reuven cannot wait, and he must sell now. If he can sell his land for half the Nisan value, he must (before taking Ma'aser Oni), for it is normal to sell for half price.

Nimukei Yosef (DH Mi): The Ramah and Rosh say that we do not discuss houses that he lives in. Even if they are worth 1000 Zuz, he need not sell them. A Mishnah in Pe'ah teaches that one need not sell his house or Kelim that he uses. Rather, he has other houses, but does not have 200 Zuz of money. He may eat Ma'aser Oni up to 100 Zuz, and sell his land for 100, for it does not decline below half the value. This is permitted even if he has 2000 Zuz of land. Even though he could sell it for 200 Zuz, he need not sell it for a loss, so he is like an Oni. When only his land declined due to his urgent need to sell, we feed him Ma'aser Oni, so he is not desperate, and can sell for the proper price. He is not an Oni, for in Nisan his property is worth 200. He is not an Ashir, for no one could sell land like his for 200 now. Therefore, he may take half from Ma'aser Oni.

Rebuttal (Chidushei Anshei Shem 5): The Rif discusses an Ashir, even if he has 2000 Zuz worth of land! Why does the Nimukei Yosef conclude like Rashi, who discusses an Oni who cannot sell all his land now for 200? What is the Nimukei Yosef's source that we give him up to 100 of Ma'aser Oni? However, the Beis Yosef said that the Rambam holds like this.

Rambam (Hilchos Matanos Aniyim 9:16): If one has houses, field and vineyards, and if he would sell them now in winter he must sell them cheaply, but if he would wait until summer he could sell for the proper price, we do not force him to sell. We feed him Ma'aser Oni up to half their value. He need not force himself to sell outside the selling season.

Rambam (17): If others sell for high prices, but he cannot because people know that he must sell, we do not force him to sell. He eats Ma'aser Oni until he sells for the right price and all will know that he is not desperate to sell.

Kesef Mishneh: R. Yerucham explains the Rif to say that it is abnormal that one cannot sell for half the price. If he cannot, we assume that it is because people know that he needs to sell, so we feed him Ma'aser Oni until he can sell for half price. The Rambam is like the Rif, except that the Rif explains 'we feed him until half' to mean until he finds one who will pay half the value. The Rambam explains that we give to him Ma'aser up to half the value of his land, for presumably it does not sell for less than half. It is difficult to say that the Rambam means like the Rif. The Rambam omitted the case when everyone's land became cheaper, for it is obvious.

Hagahos Ashri (Bava Kama 1:2): Rashi explains that if one has 200 Zuz, he may not take Leket, Shichchah and Pe'ah. If he does not have 200 Zuz, even if he is given 1000 Zuz at once, he may take it. If he has houses and field but cannot sell them due to his negligence, that people know that he must sell, we do not give anything to him. If he was not negligent, even if they are worth more than 200, he may take before selling them for less than their value, even if others' land also declined. The Rif says that if everyone's property declined, we do not give to him, provided that it is still worth 200. If only his land declined, because people know that he must sell, even if he could sell for slightly less than it is worth, we give to him Ma'aser Oni.

Ra'avad (7a DH Harei): Even though he sells now in Nisan, and he can only get half the usual Nisan price, since this is the usual price in Tishrei, he may not receive Ma'aser Oni.

Poskim

Shulchan Aruch (YD 253:3): If Reuven has land that is worth less in winter and more in summer, we do not force him to sell. We feed him Ma'aser Oni up to half their value. He need not force himself to sell outside the selling season.

Rema: Some say that we feed him only until he can sell them for half value.

Shulchan Aruch: If others bought for high prices, but he cannot because people know that he must sell, we do not force him to sell. He eats Ma'aser Oni until he sells for the right price and all will know that he is not desperate to sell.

Shach (5): The Shulchan Aruch copied the Rambam's Halachah, and holds like the Rif's text. If he cannot sell his land now for 200, he is a full Oni, and may receive even 1000 Zuz! We feed him up to half the value of his land. If he cannot sell for half, we feed him more. Tosfos, Semag and R. Yerucham explain that the Rif holds that we feed until he can sell for half the value in Nisan.

Gra (7*): There are four explanations. The first opinion in the Shulchan Aruch is like the Rif, and like the Rambam regarding 'until half' (we give to him up to half his land's value). The Rema is like Tosfos, who explains that we give to him until he can find one to pay half the value of his land. This is like Rashi's second Perush, except that Rashi reverses the laws of when everyone's land declined and only his declined. Rashi's first Perush is like the Rambam, except that he explains that it refers to an Oni, and that he may get Ma'aser Oni unless he was negligent.. There is another Perush of the Rif (that he does not receive if everyone's land is cheaper and he could sell for 200, but he may receive if he cannot sell for as much as other people would sell for).

Beis Yosef (DH Kosav): R. Yerucham says in the name of R. Yonah that when we give to him up until half, we give what he needs each day. Regarding a true Oni, we may give 1000 Zuz at once.