1. Selling a בהמה גסה to an idolator is forbidden because of נסיוני (and שכירות and שאילה)

The Mishnah taught it is forbidden everywhere to sell בהמה גסה – large livestock to an idolator. The Gemara explains that we are concerned he will do מלאכה with it on Shabbos. The Gemara asks that this should be permitted, since the idolator fully acquired the animal, and even if he merely borrowed or rented it, it should be considered acquired by him, and its מלאכה is permitted!? Rami brei d’Rav Yeiva answered: גזירה משום נסיוני – it is a decree because of testing an animal before the sale. Sometimes a Jew may sell an animal close to Shabbos, and the non-Jewish buyer will ask to test the animal’s ability to carry. The animal will hear its owner’s voice and walk with its burden, which pleases the seller, and he thus violates מחמר אחר בהמתו בשבת – walking after his ladened animal on Shabbos.

The Gemara proves, from the fact that a Kohen who rents a cow from a Yisroel cannot feed it terumah, that renting or borrowing does not acquire the item. Therefore, it concludes that selling a בהמה גסה to an idolator is forbidden because of a גזירה that he may rent or lend it to him (and its מלאכה would be prohibited), as well as the concern of testing.

1. Can one assume the idolator bought an animal for slaughter?

Rav Huna once sold a cow to an idolator, and explained to Rav Chisda why this was permitted: אימור לשחיטה זבנה – say [the idolator] bought it for slaughter, not for work (the Mishnah’s ban would apply to animals not commonly eaten (e.g., horses), and are purchased for work, or any whenever the idolator says his purchase is for work). This leniency can be demonstrated from Beis Hillel, who permits selling a field during shemittah (to a Yisroel suspect of violating shemittah), although the seller is responsible for work performed in his field during shemittah, because the buyer may let it lie fallow.

Rav Ashi ultimately concludes with a rule: כל היכא דאיכא למיתלא תלינן – wherever there one can make a reasonable assumption for a leniency, we make [that] assumption, ואף על גב דמצווה – even though he is commanded regarding that property’s work. If there is no reasonable assumption for leniency, he cannot even sell something which is not his own responsibility, because of לפני עור for the buyer (e.g., selling farming tools to someone suspected of violating shemittah).

1. Selling to a Yisroel who is suspected of selling to idolators

Rabbah sold a donkey to a Yisroel who was suspected of selling such animals to idolators. Abaye asked him why he did so, since this Jew may sell the animal to an idolator, to which Rabbah replied: לעובד כוכבים קא מזבין – “To an idolator he will sell it, לישראל לא קא מזבין – but to another Yisroel he will not sell it?!” We should leniently assume he will sell it to a Yisroel. This is challenged from a Baraisa which prohibits selling animals to כותים, although they are not suspected of relations with animals, and another Baraisa prohibits selling any weapons to כותים, although they are not suspected of murdering Jews. This proves that the ban on selling these items to כותים is because they will sell them to idolators!?

Although one might argue this only applies to כותים, who do not repent, but not to suspected Jews who do repent, Rabbah bar Avuha explicitly said that just as it is forbidden to sell certain items to an idolator, כך אסור למכור לישראל החשוד למכור לעובד כוכבים – it is similarly forbidden to sell such items to a Yisroel suspected to sell them to an idolator. Upon hearing this, Rabba ran three פרסאות (or, one פסרה in the sand) to buy back the donkey, but did not catch the purchaser.