Gittin 23B

מכלל דעבד כשר. אמר רב אסי, אמר רבי יוחנן: אין העבד נעשה שליח לקבל גט לאשה מיד בעלה, לפי שאינו בתורת גיטין וקדושין.

one can learn by inference that a slave is fit. Rav Asi says that R' Yoḥanan says: A slave cannot become an agent to receive a bill of divorce for a woman from the hand of her husband, because he is not included in the halakhot of divorce and betrothal, and one can act as an agent only in a matter that applies to him.

מתקיף לה רבי אלעזר: טעמא במילתא דליתיה, הא במילתא דאיתיה ⁠– כשר?!

R' Elazar objects to this explanation as to why a slave cannot act as an agent: The reason that a slave is unfit is that this agency pertains to a matter whose halakhot he is not included in, but for a matter whose halakhot he is included in, i.e., a mitzva that applies to a slave, is he fit to serve as an agent?

והא גוי והא כותי; דאיתנהו בתורת תרומה דנפשייהו, דתנן: הגוי והכותי שתרמו משלהם ⁠– תרומתם תרומה; ותנן: גוי שתרם של ישראל, אפילו ברשות ⁠– אין תרומתו תרומה!

But what of a non-Jew and a Samaritan, who are included in the halakhot of teruma with regard to their own produce, i.e., they must designate a portion of it for the priest, as we learned in a Mishnah (Terumot 3:9): With regard to a non-Jew and a Samaritan that separated teruma from their own produce, their teruma is considered teruma. And yet we learned in a different Mishnah (Terumot 1:1): In the case of a non-Jew who separated teruma from a Jew’s produce, i.e., acted as his agent, even if he did so with permission from the Jew, his teruma is not teruma.

מאי טעמא? לאו משום דכתיב: (אתם) ״גם אתם״⁠ – מה אתם ישראל, אף שלוחכם ישראל?!

What is the reason for this? Is it not because it is written “you” in the verse that is the source for the halakhot of agency: “So you also should set apart a gift to YHWH of all your tithes” (Numbers 18:28), and the rabbis expound the expression “so you also” to mean the following: Just as you, the ones appointing the agents, are Jews, so too, your agents must be Jews. Since slaves are not full-fledged Jews, they should be disqualified from ever acting as agents, even in a matter in which they are included in its halakhot.