Gemara

51b (Mishnah) Question: How much can a Sela lack, and one (who gives it for a Sela) does not transgress Ona'ah?

Answer #1 (R. Meir): It can lack up to four Isarim, one Isar per Dinar;

Answer #2 (R. Yehudah): It can lack up to four Pundeyonim, one Pundeyon per Dinar.

Answer #3 (R. Shimon): It can lack up to eight Pundeyonim.

(If a coin lacks too much), in a big city (in which there are moneychangers), one may return it until the time needed to show it to a moneychanger. In a village, one may return it until Erev Shabbos.

If the one who gave the coin recognizes it, he accepts it even after 12 months. He only has complaints.

It may be used to redeem Ma'aser Sheni, for only an irritable person refuses to accept it.

Question: Why do the Tana'im argue only about Ona'ah of a coin? They do not argue about a purchase (in the Mishnah 49b)!

Answer #1 (Rava): They argue also about a purchase. That Mishnah is R. Shimon, who says that one part in six is Ona'ah.

Answer #2 (Abaye): Regarding a garment, one pardons up to a sixth. If needed, one overpays for clothing. However, if people will not accept a coin, one who received it is not appeased!

(Beraisa): An Isar less than this, it is forbidden.

(Abaye): This means that if it lacks an Isar more than the allowed deficiency, one may not use it.

Rejection (Rava): If it lacks anything more than the allowed deficiency, one may not use it! Rather, if a Sela lacks an Isar per Dinar, one may not use it. The Beraisa is R. Meir.

Rishonim

Rif: The Halachah follows Rava, therefore, the Halachah follows R. Shimon regarding Ona'ah of a Sela, for the Stam Mishnah.of a Talis is like him. Also, Rava (50b) rules like R. Shimon regarding a Talis.

Gra (CM 227:23): The Halachah need not follow the Stam Mishnah, for is followed by a Mishnah in which Tana'im argue! We may learn from Rava's ruling like R. Shimon.

Rosh (4:19): We never find Rava's opinion before Abaye, for Abaye was Rosh Yeshivah before Rava, and he would begin (the discussion) in the Beis Medrash. We must say that the text says that Rabah argues with Abaye. The Halachah follows Abaye, for he is Basra.

Piplulei Charifta (6): Abaye does not learn Ona'ah of coins from a garment. The general rule is, the Halachah follows R. Yehudah against R. Meir or R. Shimon

Prishah (CM 227:16): Abaye holds like R. Yehudah.

Hagahos Ashri: The same applies to a (coin called a) Pashut.

Question (Hagahos Eretz Tzvi): What is the Chidush? Why should we think that the Halachah is different for different coins?!

Rosh (20): This (a deficient coin may be used to redeem Ma'aser Sheni, since a proper person accepts it) is when it is accepted with difficulty.

Rambam (Hilchos Mechirah 12:9): If a gold Dinar was 24 silver Dinarim, and one made a Dinar worth 20 or 28 Dinarim, he returns the Ona'ah. If it was more than this, the mintage is Batel. If it was less, it is pardoned.

Rambam (10): The same applies if a Sela was lacking a sixth, and one paid a fixed number of them, not for their weight. He returns the Ona'ah.

Magid Mishneh: If one paid a fixed weight, he returns a lack of any amount. The Ramban and others say that the text says 'Rabah', and rule like R. Yehudah, that one part in 12 in Ona'ah. It is unlike Ona'ah of other things, like Abaye explained.

Rambam (11): One may return a coin in a big city until the time to show it to a moneychanger. In a village one may return it until Erev Shabbos, for moneychangers are scarce and only they know a Sela and what it lacks.

Rambam (12): If Levi gave a deficient coin to Ploni and Levi recognizes it, he exchanges it even after 12 months. If it could be spent with difficulty, Levi need not exchange it, unless he wants to fulfill Midas Chasidus.

Magid Mishneh: The Rambam connotes that if it cannot be spent even with difficulty, Levi is obligated to exchange it even after the time. The Gemara connotes that after the time, it is always only Midas Chasidus.

Tosfos (51a DH Isar): Even though any mistake about measure, weight or number is returned, the coin was given for a coin, not for its weight.

Poskim

Shulchan Aruch (CM 227:16): If a gold Dinar was 24 silver Dinarim, and one made one of 20 or 28, he returns the Ona'ah. If it was more than this, the mintage is Batel. If it was less, it is pardoned. Likewise, a Sela lacked a sixth, and one paid a fixed number of them, not based on their weight, he returns the Ona'ah. If one bought for a fixed weight, he returns a lack of any amount.

SMA (27): For anything sold by measure, weight or number, Ona'ah of any amount is returned.

Rema: Some say that Ona'ah for coins is one part in 12. Less than this is pardoned; more is Bitul Mekach (the sale is void). Whenever it lacks an amount that is pardomed, one may spend it l'Chatchilah like a good coin (Sa'if 6).

Tosfos (52a DH Omar): Abaye/Rava explain the Beraisa to forbid using coins missing an Isar/any amount more than the Shi'ur Ona'ah, even with intent to return the Ona'ah, for it is a Mekach Ta'os. If it lacks the Shi'ur Ona'ah, one may use it with intent to return the Ona'ah if the other party will demand it within the time. If it lacks less than the Shi'ur Ona'ah, one may use it without intent to return anything, for it is pardoned. Alternatively, Abaye forbids even to spend it for its value. Chachamim are not stringent for a small loss up to an Isar. Rava disagrees.

Shulchan Aruch (17): One must exchange a Dinar or Sela that he gave in a big city until the time needed to show it to a moneychanger. In a village, moneychangers are scarce. One may return it until Erev Shabbos, when he comes to spend it, for only moneychangers know a Sela and what it lacks.

SMA (30): Even if one is rich and has other coins to spend, Chachamim did not give him more time, for normally if one has coins that he is unsure about, he spends them and saves the coins that are surely good.

Shulchan Aruch (ibid): After this time, the one who shortchanged need not exchange it. However, it is Midas Chasidus to return it even after this time if he recognizes it. This is only if it could be spent with difficulty.

SMA (31 and Prishah DH u'Bilvad): The last words mean that if it cannot be spent even with difficulty, one need not return it even for Midas Chasidus. Even one who sold for twice the value need not return the Ona'ah after the time to show it to a merchant. Ir Shushan explains that if it cannot be spent with difficulty, one must even after a long time. This is wrong. The Rambam connotes like this, but the Magid Mishneh said that the Gemara connotes otherwise. This is why the Shulchan Aruch did not adopt the Rambam's words, like usual, rather, the Tur's words. The Ir Shushan explains the Tur like the Rambam; this is wrong.

Taz: The Ir Shushan is correct. The Seifa holds that a proper person accepts the coin, i.e. for one can spend it with difficulty. The Rambam infers that the Reisha, which limits the time to return it, also discusses such coins. The Tur agrees that after the time it is Midas Chasidus to accept it, but one must always accept a coin that cannot be spent even with difficulty. The Magid Mishneh holds that the Mishnah discusses a coin that cannot be spent even with difficulty, and even so it is only Midas Chasidus to return it after the time. This is no support for the SMA!

Gra (25): The Rambam holds that the Seifa (a proper person would accept it) applies also to the Reisha (a coin missing the Shi'ur of Ona'ah). This is difficult, for one may not keep such a coin (Sa'if 18)!