Gittin 38B

וכפו את רבה ועשאה בת חורין, ואמר רב נחמן בר יצחק: מנהג הפקר נהגו בה!

and the court forced her master to emancipate her, and he made her a free woman. And Rav Naḥman bar Yitzḥak said in explanation of why they forced him to do this: They took liberties with her, i.e., people had sex with her freely. This demonstrates that it is permitted to free a slave to prevent people from violating prohibitions.

הכי השתא?! התם לא לעבד חזיא ולא לבן חורין חזיא, הכא אפשר דמיחד לה לעבדיה, ומנטר לה.

The Talmud rejects this proof: How can these cases be compared? There, in the case of a half-female slave half-free woman, she is not fit for marrying a slave and she is not fit for marrying a freeman. This is why she is available to all, and the only way to solve this problem is to emancipate her. Here, in the case of the female slave, it is possible for the master to assign her to marry his slave, and that slave will guard her from people who wish to be promiscuous with her. Therefore, it is not necessary to emancipate her.

גופא ⁠– אמר רב יהודה אמר שמואל: כל המשחרר עבדו עובר בעשה, שנאמר: ״לעולם בהם תעבודו״.

§ The Talmud returns to discussing the matter itself cited above. Rav Yehuda says that Shmuel says: Anyone who emancipates his slave violates a positive mitzva, as it is stated: “Of them may you take your bondmen forever” (Leviticus 25:46). This is a positive mitzva requiring that one subjugate slaves their entire lives. Therefore, it is prohibited to emancipate them.

מיתיבי: מעשה ברבי אליעזר שנכנס בבית הכנסת ולא מצא עשרה, ושחרר עבדו והשלימו לעשרה! מצוה שאני.

The Talmud raises an objection from a baraita: There was an incident involving R' Eliezer, who entered a synagogue to pray, and he did not find a quorum of 10 men, and he emancipated his slave and had him complete a quorum of 10. This demonstrates that one is permitted to emancipate his slave. The Talmud answers: Freeing a slave to enable the performance of a mitzva, e.g., completing a quorum, is different. This does not demonstrate that in general one is permitted to emancipate his slave.