1) WHO IS "HILLEL" QUOTED BY THE GEMARA?
OPINIONS: The Gemara cites Hillel who asked a question to Rebbi. This Hillel cannot be the same Hillel as the leader of the academy of Beis Hillel, because that Hillel preceded Rebbi by six generations (Rebbi was his great-great-great-great grandson). Who is the Hillel quoted by the Gemara?
(a) RAV YAKOV EMDEN writes that this Hillel may be Rebbi's great-great-grandson (who was the son of Rebbi Yehudah Nesi'ah the son of Raban Gamliel the son of Rebbi). Rav Yakov Emden mentions incidentally that the Hillel mentioned in the Gemara here is the last known direct paternal descendant of the house of the original Hillel. However, he mentions that in SEFER SIVUV REBBI PESACHYAH it is recorded that there was a man in Eretz Yisrael named Rebbi Nehora'i who claimed that he was a direct paternal descendant of the house of Hillel.
(b) RAV YITZCHAK ISAAC CHAVER explains that the correct text must read that Hillel asked Rava a question, not Rebbi. He appears to reject the possibility that Rebbi lived long enough to have his great-great-grandson ask him a question.
Historical records support Rav Yitzchak Isaac Chaver's explanation. According to HA'LEVI, Rebbi died in the year 3940 (192 CE). The great Hillel (who calculated and established the Jewish calendar used today) to whom Rav Yakov Emden refers was still alive in the year 4119 (359 CE; see ArtScroll's "History of the Jewish People: From Yavneh to Pumbedisa," pages 108 and 194). Even if Hillel was a young child and asked Rebbi his question on the day Rebbi died, this means that Hillel lived at least 167 years (and longer, since he presumably lived beyond the year 359 CE), and this is unlikely. (Y. MONTROSE)
2) MAY THE BUYER CANCEL THE TRANSACTION WHEN HE WAS OVERCHARGED BY ONE SIXTH?
OPINIONS: The Mishnah teaches that when a seller told a buyer that he is selling him good wheat, and he actually sold him bad wheat, the buyer may cancel the entire sale. This law seems to be in accordance with the Mishnah earlier (51a). However, in the Gemara earlier (50b), Rava states that when a buyer is overcharged by one sixth of the fair market value of the merchandise, the sale is binding, but the seller must give back the amount of money which he inappropriately overcharged. Does the Mishnah here contradict Rava's statement?
The RASHBAM (DH Machar) explains that Rava understands that the Mishnah earlier (51a) follows the view of Rebbi Yehudah ha'Nasi, which is not the Halachic opinion. Rava rules like the opinion of Rebbi Nasan. However, Rava still agrees with the Halachah stated in the Mishnah, even when the amount overcharged is one sixth. This is because there is a fundamental difference between the typical case of Ona'ah and the case of the Mishnah. In an ordinary case of Ona'ah there is only one element of fraud: the price is one sixth higher than the fair value. As long as the price is rectified, there is nothing wrong with the sale. In the case of the Mishnah, there are two elements of fraud: the price, and the fact that the merchandise is not the type of merchandise the buyer expected. In such a case, Rava agrees that the buyer has the right to invalidate the sale altogether.
This is how the Rishonim understand the Mishnah. However, the Gemara opens with a general statement of Rav Chisda 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. Rav Chisda seems to refer to an ordinary case of overcharging, since he does not mention anything about the quality of the merchandise. Accordingly, his view conflicts with Rava's opinion that a buyer may not invalidate a sale in a case of ordinary Ona'ah, but he may merely demand the amount that he was cheated from the seller. Does Rav Chisda argue with Rava?
(a) The RASHBAM (DH Amar Rav Chisda) explains that Rav Chisda indeed argues with Rava and rules like the Mishnah earlier (51a), which, according to Rava, expresses the view of Rebbi Yehudah ha'Nasi. This is also the opinion of the RIF.
(b) The RASHBA and RA'AVAD quote the GA'ON who explains that Rava and Rav Chisda do not argue. Rav Chisda refers to a case in which the buyer did not yet do an act of Meshichah on the merchandise; he merely paid money for it. The Halachah is that if a buyer pays money but does not accept the goods, he technically is allowed to cancel the deal; he merely is subject to a curse of "Mi she'Para" (see Bava Metzia 44a). Rav Chisda is teaching that even though this is the Halachah in most cases, in a case where the seller cheated the buyer by the amount of one sixth the buyer may revoke the sale as long as he did not yet do Meshichah, and there is no curse of "Mi she'Para." Rava, on the other hand, refers to a case in which the buyer already did a proper Meshichah. Rav Chisda agrees that in such a case the buyer cannot retract the sale. (Y. MONTROSE)