The Mishnah states that one is permitted to send the thigh of an animal to a non-Jew, even if the Gid Hanasheh (sciatic nerve) remains within it. This is because there is no concern that the non-Jew will, in turn, sell the thigh to a Jew, who might then consume it without removing the Gid Hanasheh. The reasoning is that the location of the Gid Hanasheh is noticeable when the thigh is whole, allowing the non-Jew to recognize its presence.
However, Rabbi Yehuda offers a crucial distinction. He asserts that this permission applies only when the thigh is a full, uncut piece of meat. In such a case, the Gid Hanasheh is clearly visible, and anyone who purchases it, even a non-Jew, would be aware of its presence. But if the meat is cut up, one may not send it to a non-Jew without first removing the Gid Hanasheh. This is because, once cut, the Gid Hanasheh is no longer easily noticeable. Should the non-Jew then sell this cut meat to a Jew, the Jew might not realize the Gid Hanasheh is still inside, leading to a transgression.
The Gemara then delves into the circumstances under which a Jew might purchase meat from a non-Jew. It explores two primary scenarios concerning the sale of non-kosher meat, specifically a tereifah (an animal with a defect that renders it non-kosher), by Jewish butchers to non-Jews.
One scenario is a place where butchers do not make an announcement when an animal is a tereifah. In such a town, all Jewish butchers sell their tereifot directly to non-Jewish butcher shops without publicizing the animal's non-kosher status. In this situation, a Jew is never permitted to buy meat from a non-Jewish butcher shop. Even though all the slaughterers in the town are Jewish, there's a concern that the meat sold by the non-Jewish butcher might be a tereifah that was sold to them by a Jewish butcher. Since no announcement is made, Jews would not know to avoid it, and might inadvertently purchase and consume non-kosher meat. Therefore, in such a place, one cannot even send cut meat to a non-Jew, because no Jew would buy it anyway; all meat from non-Jewish shops is presumed problematic.
The other scenario is a place where butchers do make an announcement when an animal is a tereifah. On days when an announcement is made, Jews know that a tereifah was found and sold to a non-Jew, and they avoid purchasing meat from non-Jewish butcher shops. However, on days when no announcement is made, it is understood that all meat in the non-Jewish butcher shops is kosher, because all slaughterers in the town are Jewish, and any issues would have been announced. In such a place, if Jews are permitted to buy from non-Jewish butcher shops on days without announcements, then one should logically not be allowed to sell even whole meat to a non-Jew, as the non-Jew might cut it up and sell it to a Jew, who would then buy the cut meat without realizing the Gid Hanasheh is present. This leads back to the original question: why does the Mishnah permit selling whole meat to a non-Jew?
The Gemara offers several answers. One explanation is that in a place where announcements are made, Jews know the distinct style in which non-Jews cut meat. If meat is cut by a non-Jew, a Jew would recognize it and therefore be on guard regarding the Gid Hanasheh. This means that if the meat is whole, the Gid Hanasheh is noticeable. If it is cut, the distinct non-Jewish cutting style serves as a warning. Therefore, one can sell whole meat to a non-Jew without concern, even if the non-Jew cuts it up, because a Jew would recognize the non-Jewish cut and would know to check for the Gid Hanasheh. This addresses the concern about the Gid Hanasheh, while the announcement system addresses concerns about tereifot.
Another answer, applicable to places where no announcements are made, is that since Jews do not buy any meat from non-Jews in such locations, there's no concern about a Jew inadvertently purchasing meat with the Gid Hanasheh. However, the Gemara introduces a new prohibition: one still cannot sell cut meat to a non-Jew. This is due to a gezeirah (rabbinic decree) to prevent a potential misunderstanding. If a Jew is seen giving cut meat to a non-Jew in the presence of another Jew, the observing Jew might assume that since the meat is cut in the Jewish style (which typically implies the Gid Hanasheh has been removed) and is being given by a Jew, it must be kosher and free of the Gid Hanasheh. This could lead the observing Jew to eat meat with the Gid Hanasheh. Therefore, in places without announcements, only whole meat may be sold to a non-Jew, as the presence of the Gid Hanasheh is obvious in a whole piece, preventing such a misunderstanding.
A third answer, offered by Ulla, is based on the principle of geneivat da'at, which prohibits misleading or "stealing the mind" of others, even non-Jews. If a Jew sells cut meat to a non-Jew without removing the Gid Hanasheh, the non-Jew might mistakenly believe that the Jew, out of kindness or respect, has already performed the laborious task of removing the Gid Hanasheh. This would cause the non-Jew to feel undeservedly indebted to the Jew, which constitutes geneivat da'at. This concern is particularly relevant in places where no announcements are made, as Jews would not be buying from non-Jews anyway, so the issue is not a Jew eating the meat, but the non-Jew being misled. If whole meat is sold, the Gid Hanasheh is visible, so the non-Jew is not misled. But with cut meat, the non-Jew might assume the removal has been done, creating a false impression of favor.
The Gemara then explores the source for the prohibition of geneivat da'at, citing Shmuel. It is related that Shmuel was on a boat and instructed his servant to pay the ferry operator. The servant, instead of money, gave the non-Jewish ferry operator a chicken that had become a tereifah. Shmuel became angry. According to one interpretation, Shmuel was angry because the non-Jew would assume the chicken was properly slaughtered (a shechita) and kosher, thus being misled into thinking he received a valuable, kosher item when it was actually a non-kosher one. This is an act of geneivat da'at.
A second version of the story by Rava states that Shmuel told his servant to give the ferry operator an anpaka (a vessel for undiluted wine). The servant, however, gave diluted wine in the anpaka. Shmuel was angry because the non-Jew, seeing the special cup, would assume he was receiving undiluted wine, thereby being tricked. This, too, is an instance of geneivat da'at.
The Gemara questions the strength of this proof for Shmuel's stance on geneivat da'at, as his anger was not explicitly stated to be for that reason. In the first story, Shmuel might have been angry that the servant kept a tereifah in his possession for too long. In the second, he might have been angry that the servant disobeyed his instruction to give undiluted wine in the anpaka, regardless of what the non-Jew thought. Since Shmuel did not explicitly state his reason, the proof for geneivat da'at from these stories is not conclusive.
The Gemara provides other examples of geneivat da'at: 1. Inviting a friend for a Shabbat meal when you know he is unable to come. The friend will feel indebted, unaware that you knew he couldn't accept. 2. Offering gifts you know a person will not accept, causing them to feel obligated to you. 3. Opening a barrel of wine for a guest that has already been sold to a shopkeeper. The guest will think you are opening a new barrel specifically for them, incurring a loss for their honor, when in reality you are not losing anything. This is permitted only if you explicitly state that the barrel is already sold and you are opening it anyway. 4. Offering oil from an empty jug to someone you know won't take it. The person will feel you are being generous, when you are merely pretending.
However, the Gemara clarifies that if these actions are performed to honor the guest in front of others, to elevate their perceived status, it is permissible. For instance, opening a "new" barrel of wine or offering oil from an "empty" jug in public, to show others that the guest is an important person, is allowed. The intent here is to benefit the guest by enhancing their reputation, not to personally mislead them for your own gain. But if it's just between you and the guest, without this public benefit, it is forbidden.
The Gemara then notes a contradiction: Ulla was invited to Rabbi Yehuda and opened barrels that had already been sold. This seems to contradict the prohibition. The resolution is that Ulla was different; he loved his guests so much that he would have opened a new barrel even if the existing one hadn't been sold. It just happened to be sold. This implies that his intent was genuine generosity, not misleading. The Gemara concludes by noting that once a barrel is opened, it spoils, which is why opening a sold barrel is usually problematic unless the intent is pure or the fact of its sale is disclosed.