Avodah Zarah, Chapter One, Mishnah Six
This mishnah discusses the prohibition of a Jew from selling animals to non-Jews, lest the non-Jews use them for work on the Sabbath.
1) In a place where it is the custom to sell small domesticated animals to non-Jews, such sale is permitted; but where the custom is not to sell, such sale is not permitted.
a) In no place however is it permitted to sell large animals, calves or foals, whether whole or maimed.
i) Rabbi Judah permits in the case of a maimed one.
ii) And Ben Bateira permits in the case of a horse.
The Torah teaches on several occasions that an animal must rest on the Sabbath (see for instance Exodus 20:9). Our mishnah extends this prohibition and prohibits a Jew from selling an animal to a non-Jew, lest the non-Jew use the animal for work on the Sabbath. This is similar yet somewhat different from the issue which the mishnah discussed previously, selling potentially idolatrous objects to the non-Jew. In both cases it is forbidden to sell something to a non-Jew. However, in this case, Jews are not enjoined to prevent non-Jews from working on the Sabbath. Rather Jews are prohibited from putting Sabbath observant animals into the position where they will have to break the Sabbath. In other words this prohibition concerns the animal and not the non-Jew himself.
Not all animals are used for work. Small animals, such as sheep and goats are not used for work. Therefore in a place where it is customary to sell them to non-Jews it is permitted to do so. In other places it was customary not to sell even small animals to non-Jews, lest the Jew become confused and sell them large animals, which is prohibited in all places.
[We have not encountered many of these types of mishnahs, which permit something in a place where it is customary to do so, and forbid it in places where it is not customary. For other examples which we have learned see Bava Metzia 7:1 or 9:1.]
It is forbidden in all places to sell large animals, such as oxen and horses, to non-Jews since they will be used to perform work on the Sabbath. This prohibition includes calves and foals, even though they do not usually perform work.
Rabbi Judah allows one to sell injured animals to non-Jews since they are clearly being purchased for their meat and not in order to do work.
Ben Bateira allows the sale of a horse since horses are used for riding, which is not considered by the Rabbis to be work. Pulling plows, a work performed by oxen is considered work.
Questions for Further Thought:
Why is it forbidden in all places to sell calves and foals to non-Jews and yet there are some places that do sell small animals? Since both dont perform work why is one always prohibited and one sometimes permitted?