Avodah Zarah 15A

ורבי אלעזר אומר: אף במקום שאסרו לייחד, מותר למכור. מאי טעמא? גוי חס על בהמתו שלא תעקר. ואף רב הדר ביה, דאמר רב תחליפא אמר רב שילא בר אבימי משמיה דרב: גוי חס על בהמתו שלא תעקר.

And R' Elazar says: Even in a place where they prohibited leaving an animal in seclusion with a non-Jew, it is permitted to sell it to a non-Jew. What is the reason? Once the animal is sold to the non-Jew, there is no concern that he will engage in bestiality. This is because a non-Jew spares his own animal from bestiality, as he does not want it to become sterile through this practice. By contrast, it is prohibited to leave one’s animal in seclusion with a non-Jew, as he would have no such compunction with regard to an animal belonging to others. The Talmud notes: And even Rav retracted his opinion; as Rav Taḥlifa says that Rav Sheila bar Avimi says in the name of Rav: A non-Jew spares his animal, as he does not want it to become sterile.

ובכל מקום אין מוכרין בהמה גסה כו׳. מאי טעמא? נהי דלרביעה לא חיישינן, מעביד ביה מלאכה חיישינן.

§ The Mishnah teaches: But in every place one may not sell to non-Jews large livestock, calves, or foals, whether these animals are whole or damaged. The Talmud explains: What is the reason? The Talmud explains: Granted, we are not concerned about the non-Jew engaging in bestiality with the animal, but we are concerned about him putting the animal to work on Shabbat.

וניעביד, כיון דזבנה קנייה! גזירה משום שאלה ומשום שכירות.

The Talmud expresses puzzlement: And let the non-Jew put it to work. Why should one be concerned about this possibility? Since he bought it, he acquires it and may put it to work on Shabbat, as it no longer belongs to the Jew. The Talmud answers: Selling it is prohibited by rabbinic decree due to the concern of lending and due to the concern of leasing the animal to the non-Jew, as in those cases the animal would be performing work on Shabbat when it is owned by a Jew.