1) HALACHAH: "ONA'AH" FOR COINS
OPINIONS: The Mishnah (51b-52a) discusses how much a Sela coin may be deficient in silver (as a result of constantly being handled) and not constitute a problem of Ona'ah. Rebbi Meir says that it may be lacking up to 1/24 of its proper weight in silver. Rebbi Yehudah says that it may be lacking up to 1/12. Rebbi Shimon says that it may be lacking up to 1/6, the same measure of Ona'ah for ordinary objects.
The Gemara asks why the Tana'im disagree only with regard to the Ona'ah of coins, but they do not disagree with regard to the Ona'ah of ordinary objects (such as a Talis). The Mishnah earlier (49b) clearly says that the Shi'ur of Ona'ah is 1/6, and no one argues.
Rava answers that there actually is no difference between ordinary objects and coins; the Tana'im disagree in both cases. The Mishnah earlier (49b) follows the view of Rebbi Shimon when it says that the Shi'ur of Ona'ah is 1/6.
Abaye answers that there indeed is a difference between merchandise and coins. A person is far more hesitant to accept coins that are deficient since he will not be able to spend them easily.
What is the Halachah?
(a) The RIF and RAMBAM (Hilchos Mechirah 12:9-10) rule in accordance with the view of Rebbi Shimon. Since Rava explains that the Mishnah earlier expresses the view of Rebbi Shimon without attributing any name to it (that is, it is a "Stam Mishnah"), the Halachah presumably follows that view, both with regard to merchandise and with regard to coins.
The NIMUKEI YOSEF asks that although the Halachah normally follows the statement of a Stam Mishnah, it does not follow a Stam Mishnah when that Mishnah is followed by a Mishnah in which the subject is subject to a Machlokes, which indeed is the case here. Why, then, do the Rif and Rambam rule like Rebbi Shimon? The Nimukei Yosef answers that this rule applies only when an Amora does not explicitly rule like the Stam Mishnah. In this case, however, Rava (50b) explicitly rules like the Stam Mishnah. This is the Halachah as recorded by the SHULCHAN ARUCH (CM 227:16).
(b) The ROSH writes that there is apparently a mistake in our text of the Gemara. It is not Rava who says that the Mishnah earlier expresses the view of Rebbi Shimon, but rather Rabah. The Rosh proves that it must be Rabah from the fact that this opinion is quoted before that of Abaye, and nowhere in the Gemara is Rava ever quoted before Abaye. Since it was Rabah who gave this answer and Abaye argues with it, the Halachah should follow the view of Abaye, as Abaye was a later Amora. Accordingly, the three Tana'im in the Mishnah here agree that the Shi'ur of Ona'ah for merchandise is set at 1/6.
Whom does the Halachah with regard to the Shi'ur of Ona'ah for coins? The Gemara in Eruvin (46b) states a rule that the Halachah follows the view of Rebbi Yehudah over Rebbi Meir and Rebbi Shimon. The Rosh therefore rules that Ona'ah of coins is 1/12, not 1/6. This is also the opinion of the RAMBAN and the REMA. (Y. MONTROSE)
2) REDEEMING "MA'ASER SHENI" WITH A DEFICIENT COIN
OPINIONS: The Mishnah (51b-52a) discusses how much a Sela coin may be deficient in silver (as a result of constantly being handled) and not constitute a problem of Ona'ah. The Mishnah concludes that "one may give such a coin to Ma'aser Sheni and not worry." When the Mishnah says that one may give such a coin to Ma'aser Sheni, does it refer specifically to a coin which is deficient more than Ona'ah, exactly Ona'ah, or less than Ona'ah?
(a) TOSFOS (DH Nosnah) says that the Mishnah refers to a coin which is deficient by the exact Shi'ur of Ona'ah.
When the Mishnah says that one may give such a coin to Ma'aser Sheni, it adds that a person who refuses to accept such a coin is considered a "bad soul." Why is this relevant to Ma'aser Sheni? The Torah states that the sanctity of Ma'aser Sheni may be redeemed only onto coins, and not onto other objects (see 54a). Since these coins are still normally accepted, one is allowed to redeem the Kedushah of Ma'aser Sheni onto these coins. The Gemara quotes Chizkiyah who explains that one is allowed to redeem the Ma'aser Sheni "b'Toras Yafah" -- "in a way that it is good." According to Tosfos, Chizkiyah means that the coin is used for Ma'aser Sheni according to its actual value, and not for the value of an actual Sela. The PNEI YEHOSHUA understands that this is also the opinion of RASHI (DH Mai Ka'amar).
(b) The RIVAN (quoted by Tosfos) also understands that the Mishnah refers to a coin which is deficient by exactly the amount of Ona'ah, but he understands that the coin is used for Ma'aser Sheni as though it has the value of a regular coin.
The RAMBAN, RASHBA, and others have difficulty with this explanation. If the coin is lacking the Shi'ur of Ona'ah, why does the Mishnah say that a person who does not take such a coin is called a "bad soul"? One is forbidden to keep such a deficient coin!
(c) Tosfos quotes the RIVAM who explains that the Mishnah refers to a coin which is only slightly deficient (less than Ona'ah). The Rivam says that this explanation is more accurate, because "b'Toras Yafah" literally means that it is considered a good coin. Just as one normally redeems onto a good coin an amount of Ma'aser Sheni which is slightly less than the coin's total value, one may also act this way with a coin which is slightly deficient. This is also the opinion of the ROSH.
(d) The RAMBAN, RASHBA, and others also understand that the Mishnah refers to a coin which is missing less than a Shi'ur of Ona'ah of its value, but they maintain (like the first explanation of Tosfos) that "b'Toras Yafah" means that the coin is used for Ma'aser Sheni according to its actual value. (Y. MONTROSE)