1) MAY A SELLER CANCEL THE TRANSACTION WHEN HE OVERCHARGED THE BUYER BY ONE SIXTH?
OPINIONS: Rav Chisda states that when a seller sells an object for six when it is really worth five, and then the object increases in value to eight, the only person who may invalidate the original sale is the buyer, not the seller. The seller's act of cheating certainly should not allow him to revoke the profit of the buyer. According to this logic, the same Halachah should apply in a case where the seller overcharges the buyer by more than one sixth of the fair value. There, too, he should not be allowed to benefit from his act of cheating by canceling the sale and profiting from the object's increased value. However, the Gemara in Bava Metzia (50b) states that if a seller overcharges more than one sixth of the object's fair value, the sale is invalid. This implies that the sale is automatically invalid, even in a case where the dishonest seller stands to profit as a result. What is the Halachah in such a case?
(a) TOSFOS (DH Iy Lav) explains that the sale indeed is automatically invalid when the seller overcharges by more than one sixth, even though the seller benefits as a result. In order to be considered a sale, there must be some validity to the price. The Chachamim consider such a gross overcharge as beyond the range of a valid price, and thus the sale is invalid.
(b) Tosfos quotes the RIVAM who explains that the Gemara's logic applies even when the overcharge is more than one sixth. Accordingly, only when the buyer demands his money back is the sale invalidated. The seller cannot invalidate the sale against the will of the buyer. Presumably, the Gemara in Bava Metzia (50b) does not mention this case as an exception to the rule that the sale is invalid, because it is uncommon that a buyer will be overcharged by such a large amount and then the object will suddenly increase in value, and the buyer will end up profiting (and thus will not want to invalidate the sale).
The ROSH, who agrees with the Rivam, cites proof for this explanation. If the sale is invalidated automatically when the overcharge is more than a sixth, then essentially a seller who cheats someone by more than a sixth is better off than one who cheats by only one sixth. If a seller cheats a buyer by one sixth, the buyer cannot invalidate the sale, but can only demand that the amount of money he was cheated be returned. On the other hand, if a seller cheats a buyer by more than one sixth, there is no option for the buyer to demand the amount he was overcharged without the seller being able to invalidate the entire sale. Does this mean that the seller who cheats a buyer by more than one sixth has more rights? The Rosh writes that this obviously cannot be so. A transaction in which a buyer was overcharged by more than one sixth of the fair price is not considered a sale at all (similar to the logic of the first opinion in Tosfos cited above). However, only the buyer has the right to decide whether or not the transaction is considered a sale. (The Rosh adds that this is also the opinion of the RIF.)
The Rosh apparently understands that from the perspective of the seller, who has profited significantly (albeit unlawfully), there is no reason why the transaction should not be considered a sale (on the contrary, the seller would prefer that all of transactions be so profitable) as long as the buyer does not complain. (Y. MONTROSE)
2) ARE WINE AND VINEGAR CONSIDERED THE SAME?
OPINIONS: The Gemara records a dispute between Rebbi and the Rabanan with regard to whether wine and vinegar are classified as "Min Echad" -- "one type." Rebbi maintains that they are not "Min Echad," and the Rabanan maintain that they are.
TOSFOS (DH Yayin) asks that the Gemara later (96a) seems to side with Rebbi. The Mishnah in Terumos (2:4) teaches that one may separate Terumah from one type of fruit on behalf of fruit of the same type (grapes for grapes), but not from one type of fruit on behalf of another type of fruit (grapes for olives). The Gemara later discusses the case of the Mishnah in Terumos (3:1) in which a person separated a barrel of wine as Terumah on behalf of other barrels of wine. Later, when he opened that barrel of wine, he found that the wine had turned to vinegar. May he assume that the wine was still good at the time he separate it as Terumah (and his act of separating Terumah was valid), or must he assume that the wine had already turned to vinegar by the time he separated it as Terumah (and his act of separating Terumah was not valid)? The Gemara states that he may assume that the barrel contained wine for three days after the time that he last checked it. Hence, if he separated it as Terumah after those three days, his act of Terumah is in doubt.
Tosfos asks that this ruling seems to follow the opinion of Rebbi, who says that wine and vinegar are two separate types. Does the Gemara later indeed support the opinion of Rebbi against that of the Rabanan?
(a) The RASHBAM (96a, DH ha'Bodek) writes that the Gemara later indeed follows the opinion of Rebbi. According to the Rabanan, the Terumah is valid b'Di'eved even if it was vinegar at the time that he separated it on behalf of wine. L'Chatchilah, however, one should not separate Terumah from vinegar on behalf of wine.
(b) TOSFOS (84b, DH Yayin) explains that the Gemara later may even follow the opinion of the Rabanan. The Gemara there discusses a case of "Terumah b'Ta'us" -- "mistaken Terumah." Even according to the Rabanan, if one did not intend to separate Terumah from vinegar in order to exempt wine, his act is invalid if he discovers that what he thought was wine was actually vinegar at the time he separated it as Terumah.
Tosfos asks, however, that this explanation seems difficult based on the Gemara in Yevamos (89a). The Gemara there clearly says that if a person intended to separate Terumah from high-quality produce but mistakenly separated Terumah from law-quality produce, the Terumah is valid. The same should apply when one mistakenly separated vinegar as Terumah for wine.
Tosfos in Yevamos (DH Kishus) answers that there is a basic distinction between the two cases. In the case of two items of the same type (such as wine and vinegar), the difference is much more pronounced. In the case of two grades of quality of one type of item, the difference is more subtle. In the case of the Gemara in Yevamos, the person separated one item as Terumah for the identical type of item; it merely turned out that the item he separated was of a lower quality than the rest. In such a case, the Terumah is valid. When, however, the item he separated is found to be an entirely different item (vinegar instead of wine), even though it is of the same species, the Terumah is not valid. (Y. MONTROSE)