Pesachim 23a: Hunters or fishermen who accidentally caught non-kosher animals are allowed to sell them to gentiles. Rashi: This is only if they caught them accidentally, but deliberately doing business with non-kosher food is forbidden, as stated in the Mishnah (Sheviis 7:3). 

פסחים כג ע”א: ציידי חיה ועופות ודגים שנזדמנו להם מינין טמאין ־ מותרין למוכרן לנכריםִ.

רש”י: שנזדמנו ־ דווקא שנזדמנו דלא מצרכינן להו להפקירן, אבל לכתחלה אסור לחזר אחריהן, כדתנן התם במסכת שביעית (פרק שביעי משנה ג): אין עושין סחורה לא בנבילות וטריפות, ולא בשקצים ורמשים.

Rabbi Yehoshua Falk was asked a shailah by the Jewish owner of the kosher slaughterhouse in his town. Sometimes there was no shochet available, but the gentiles of the town wanted meat, and his gentile employees wanted work. Could he tell his employees to slaughter animals and sell them as treif?

Jewish slaughterhouses typically sell to gentiles whatever animals are declared treif, as well as the parts of the animal that Jews don’t eat. This is not considered doing business with treif, because it is an “accidental” by-product of kosher shechitah. But here he would be deliberately slaughtering non-kosher.  

The rav reasoned that although it is forbidden to do business with treif, in this case the Jew would be doing nothing, only telling his gentile employees to do the work. This would depend on the question of whether אמירה לעכו”ם (telling a gentile to do an act forbidden to the Jew) is prohibited only for Shabbos, or applies to all types of aveiros. This question is posed in Bava Metzia 90a (regarding telling a gentile to muzzle a cow while it treads out the grain). He reasoned that we can rely on the Rishonim who say that the question was unresolved, and therefore we are lenient because “telling a gentile” is a Rabbinic prohibition. However, he sent the question to his rebbe, the Chasam Sofer.

The Chasam Sofer replied that if “telling a gentile” had been relevant here, he would have agreed to permit it, both because of the opinion of the Raavad, quoted by the Rosh, that the question is unresolved and we are lenient, and also because many say that the entire prohibition of doing business with non-kosher food is Rabbinic. However, “telling a gentile” doesn’t help us here because in the end, the Jew is the business owner and he is the one making the money off the sale of the meat. The fact that he is not doing the physical work himself is immaterial.

Rather, the Chasam Sofer proposed a different solution: to sell all the animals to the gentiles and have them slaughter, sell and keep all profits for themselves.

Source: Chasam Sofer, Likutim 6:24

Pesachim 9b: If there were nine piles of matza and one pile of chometz, and a mouse came and took from one of the piles, and we don’t know which one – this is the same as the case of “the nine stores”, where we apply the rule of “kavua” and treat it as a 50-50 chance. If a piece got separated from the piles and then the mouse took it – then we follow the majority.

פסחים ט ע”ב: תשע ציבורין של מצה ואחד של חמץ, ואתא עכבר ושקל ולא ידעינן אי מצה שקל אי חמץ שקל ־ היינו תשע חנויות. פירש ואתא עכבר ושקל ־ היינו סיפא. דתנן: תשע חנויות, כולן מוכרין בשר שחוטה ואחת מוכרת בשר נבלה, ולקח מאחת מהן ואינו יודע מאיזה מהן לקח ־ ספיקו אסור. ובנמצא הלך אחר הרוב.

One day in 1949, a woman in Jerusalem went to her freezer to get some margarine to fry schnitzel in. In the freezer there were four identical packages. She took from one at random and used it. Later, she remembered that three of the packages were margarine and one was butter. She rushed to ask Rabbi Tzvi Pesach Frank if the schnitzel was permitted.

Rabbi Frank argued that this seems to be a case of קבוע למפרע, when at the time of the taking, the taker was unaware that one of the “stores” or sources was forbidden, but only realized this in restrospect. The Ran permits such a case, but the Rashba and Ra’ah disagree (Shach 110:14).

Combined with the Ran, we have another reason to permit it: the butter was not forbidden – the problem only began later when she cooked it with chicken – so this is not a case of קבוע. Kavua only applies to an issur. Precedent for this can be found in the Pri Megadim 110 Sifsei Daas 37, who is uncertain as to whether the rule of kavua applies to chometz before Pesach, while it is still permitted. The case of the butter is better than the chometz, for two reasons: 1) Chometz even before Pesach will forbid an endless chain of dishes and foods that touch it (נ”ט בר נ”ט דאיסורא) whereas butter will only affect the first item in the chain, not the second (נ”ט בר נ”ט דהתירא). 2) Chometz is forbidden to nullify even before Pesach as per the rule that אין מבטלין איסור לכתחילה, whereas milk is permitted to nullify in water and later mix into meat (Tzlach in Beitzah).

Furthermore, even the Rashba and Ra’ah, who apply kavua even retroactively, only forbid it as a 50-50 safek. In the case of the schnitzel, the entire issue would only be a Rabbinic prohibition of chicken with milk. Therefore we can apply the rule of תולין, brought in Yoreh Deah 111, that when there are two possibilities of what could have fallen in, with Rabbinic prohibitions we assume the permitted one fell in. However, the pan should be kashered because it is a דבר שיש לו מתירין – there is a way to render it permitted.

Source: Har Tzvi Yoreh Deah 99

Pesachim 50b: If one goes from a place where they work on Erev Pesach morning to a place where they do not work, or vice versa, he must follow the stringencies of both places.

פסחים נ ע”ב: ההולך ממקום שעושין למקום שאין עושין או ממקום שאין עושין למקום שעושין נותנין עליו חומרי מקום שיצא משם וחומרי מקום שהלך לשם.

שו”ע או”ח תצ”ו ס”ג: בני ארץ ישראל שבאו לחוצה לארץ אסורים לעשות מלאכה ביום טוב שני בישוב אפילו דעתו לחזור, וכל זמן שלא הגיע לישוב אפילו אין דעתו לחזור מותר לפי שעדיין לא הוקבע להיות כמותן אבל אם הגיע לישוב ואין דעתו לחזור נעשה כמותן ואסור בין במדבר בין בישוב וכל חוץ לתחום אין נותנין עליו חומרי מקום שהלך לשם.

The Shulchan Aruch (Orach Chaim 496:3) rules that keeping one day Yom Tov in Eretz Yisroel or two days outside of Eretz Yisroel falls in the category of minhag hamakom and is subject to the rule in our Mishnah. However, the Chacham Tzvi (167) disagrees. He writes that minhag hamakom applies only in questions of halacha, where the same halacha applies in all locations, yet different communities follow different opinions. For example, the Gemara says that Eretz Yisroel and Bavel followed two different practices as to whether a certain fat is permitted. What is forbidden fat is forbidden everywhere, but certain places had a minhag to follow the opinion that considers certain fats not forbidden. If the community that considered it forbidden were to move en masse to the other location, they would continue to refrain from eating it, because the prohibition is not dependent on location. But Yom Tov Sheini is not a universal halachic question; it simply depends on whether the messengers of Beis Din reached that particular place. Therefore, people from Chutz Laaretz who are staying temporarily in Eretz Yisroel should only keep one day Yom Tov, just as they would if they had visited during the time when Beis Din sent out messengers.

When the Satmar Rav lived in Eretz Yisroel for a whole year in 1945-46, he kept only one day Yom Tov. After moving to the United States, he made four visits to Eretz Yisroel (in 1952, 1955, 1959 and 1965). He was always careful not to be in Eretz Yisroel over Yom Tov, so as not to run into the dilemma of whether to keep one day and risk doing work on Yom Tov, or to keep two days and risk neglecting the mitzvah of tefillin. Although most poskim agree with the Shulchan Aruch, he felt that as a descendent of the Chacham Tzvi, he should be careful not to violate the Chacham Tzvi’s opinion.

Source: Rabbi Moshe Zoberman, shiur on Taanis 10b

Pesachim 46b: Rabbah said that one who cooks on Yom Tov for the next day does not violate a Torah prohibition because theoretically, guests may come today and eat the food. Although Rabbinically it is prohibited, Chazal made an exception when Yom Tov falls on Friday and one makes an eiruv tavshilin.  

Rema 527:20: One who is fasting on Yom Tov is forbidden to cook for others. R’ Akiva Eiger: This is only when one is abstaining from all food, but if one abstains from a particular food because of a chumra (e.g. kitnios), he is allowed to cook it for others, or even for himself to eat on the following day, since others could eat it today. Maharsham and Chazon Ish disagree with this in the case of kitnios but agree in the case of gebrokts.

פסחים מו ע”ב איתמר, האופה מיום טוב לחול, רב חסדא אמר: לוקה, רבה אמר: אינו לוקה. רב חסדא אמר: לוקה, לא אמרינן הואיל ומיקלעי ליה אורחים חזי ליה. רבה אמר: אינו לוקה, אמרינן הואיל. אמר ליה רבה לרב חסדא: לדידך, דאמרת לא אמרינן הואיל ־ היאך אופין מיום טוב לשבת? אמר ליה: משום עירובי תבשילין. ־ ומשום עירובי תבשילין שרינן איסורא דאורייתא? ־ אמר ליה: מדאורייתא צורכי שבת נעשין ביום טוב, ורבנן הוא דגזרו ביה, גזירה שמא יאמרו אופין מיום טוב אף לחול. וכיון דאצרכוה רבנן עירובי תבשילין ־ אית ליה היכירא.

שו”ע או”ח תקכ”ז ס”כ ברמ”א:  ומי שמתענה ביום טוב אסור לבשל לאחרים אפילו לצורך בו ביום דהוי כמי שלא הניח עירוב שאינו מבשל לאחרים (מהרי״ו).

וכתב המ”ב דהאחרונים חולקים על הרמ”א, וכתב ר’ עקיבא איגר (שו”ת א,ה) דאפילו להרמ”א היינו דוקא במי שמתענה אבל מי שנוהג איסור בדבר מסוים מותר לבשל לאחרים האוכלים אותו, או לעצמו על שבת אם אוכל אותו בשבת (כגון אשכנזי בן א”י המבשל קטניות לאחרון של פסח, הואיל וחולים או ספרדים יכולים לאכלו בו ביום(. והמהרש”ם בדעת תורה (תקכז,א) אסר דלא אמרינן הואיל ואי מיקלעי ליה אורחים כיון שמחזיק אותו כאיסור, וגם החזון איש (מט,טז) אסר מטעם אחר משום מוקצה, אבל שניהם כתבו דמותר לבשל מצה שרויה לאחרון של פסח, דאף המחמירים סוברים דמעיקר הדין מותר שהרי באחרון של פסח אוכלים, ולכך מותר לבשל ביו”ט הואיל ואם בא אורח הנוהג היתר היה יכול לאכלו בו ביום.

R’ Yaakov Kamenetsky told the following story about his great-great-great uncle, R’ Eliyahu Schick.

(R’ Eliyahu Schick’s sister was the grandmother of Chaya Shereshevsky, who married R’ Shmuel Hirsch Kamenetsky, R’ Yaakov’s grandfather.)

Once, while R’ Eliyahu was the rav of Derechin, there was a devasting fire, and he went to collect donations to help families rebuild. He came to the town of Smargon, where his cousin by marriage, R’ Leibele Shapiro (also known as R’ Leibele Kovner), who was the rav of the town, accompanied him. When they came before the home of a wealthy Chabad chassid, R’ Leibele told him there was no use going into that house because the owner would not contribute to anything in Derechin, a shtetl known to be a center of misnagdim. R’ Eliyahu himself was also personally considered to be a fervent misnaged, to the extent that Chassidim accused him of deliberately giving a psak to cause them suffering. In the year 1873, when Pesach fell on Shabbos, R’ Eliyahu prohibited the preparation of knaidlach on Friday for the last day of Pesach because he held that one may cook on Yom Tov for use on Shabbos only such food as one may eat on Yom Tov itself. When R’ Eliyahu passed away a year and a half later, the Chassidim contended that he was punished from Heaven because his ruling had prevented them from enjoying knaidlach on the single day out of Pesach when they were allowed to eat them.

R’ Eliyahu said, “I will bet you a ruble for the Volozhin Yeshiva that I can get a donation from him.” He went in and came out with three rubles. “How did you do it?” asked R’ Leibele. “I told him a story about the Alter Rebbe,” said R’ Eliyahu.

R’ Leibele then said, “You gained three rubles, and here is my ruble for Volozhin, but I cannot accompany you any longer because the Torah commands מדבר שקר תרחק.” 

R’ Leibele was a talmid of R’ Chaim Volozhiner, and his son, R’ Refoel Shapiro, was the Netziv’s son-in-law and successor as rosh yeshiva of Volozhin when it reopened in 1899. R’ Refoel’s son-in-law was R’ Chaim Brisker.

When R’ Leibele was on his deathbed, R’ Yisroel Salanter wanted to visit him, but R’ Leibel refused because he was opposed to the Mussar movement. People said to him, “Is this the time for machlokes?” R’ Leibele replied, “If not now, when? Will I not be going off in a short time to the World of Truth?”

R’ Yisroel Salanter said in his hesped on him, “The posuk in Daniel (8:12) says, ‘Truth will be cast down to the ground.’ What we are doing now is burying truth underground.” The ability to perceive one’s adversary as being truthful – while wrong – because he is consistent in his outlook indicates that R’ Yisroel himself was so unequivocally committed to truth that he had greater esteem for one who was truthful than for one who was in agreement with his Mussar approach.

[R’ Eliyahu evidently held that those who don’t eat gebrokts consider it a real prohibition and therefore cannot take into account the possibility that those who do eat it might show up for a meal – similar to the halacha of kitnios according to the Maharsham.]

Source: Making of a Godol, pages 111-114