Nedarim 36B

איבעיא להו: התורם משלו על של חבירו, צריך דעתו או לא? מי אמרינן: כיון דזכות הוא לו, לא צריך דעת. או דלמא מצוה דיליה היא, וניחא ליה למיעבדיה.

A dilemma was raised before the rabbis: In the case of one who separates teruma from his own produce on behalf of the produce of another, does he require the knowledge and consent of the owner of the produce or not? Do we say: Since it is a benefit for the other to have his produce tithed, it does not require his knowledge, as one may act in a person’s interest in his absence? Or, perhaps it is his mitzva, and it is preferable for him to perform the mitzva himself, and therefore, it does not benefit him to have the produce tithed on his behalf.

תא שמע: תורם את תרומותיו ואת מעשרותיו לדעתו. במאי עסקינן? אילימא מן בעל הכרי על של בעל הכרי, ולדעתו דמאן? אילימא לדעתו דיליה ⁠— מאן שוויה שליח?

Come and hear a proof from the Mishnah: For one for whom benefit from him is forbidden by vow, he separates his teruma and his tithes, provided that it is with his knowledge and consent. The Talmud analyzes this statement: With what are we dealing? If we say that he separates teruma from the produce of the owner of the pile for the produce of the same owner of the pile, the question is: And with the knowledge and consent of whom may he do so? If we say it is with the knowledge of the one tithing the produce, who designated him an agent to do so? One cannot tithe another’s produce unless he is designated as an agent.

אלא לדעתו דבעל הכרי. הא קמהני ליה דקעביד שליחותיה! אלא משלו על של הכרי. ולדעתו דמאן? אילימא לדעתו דבעל הכרי ⁠— הא קמהני ליה! אלא לאו, לדעתיה דנפשיה, ומשלו תורם על של חבירו. ואי אמרת צריך דעת ⁠— הא קמהני ליה! אלא לאו, אין צריך דעת.

Rather, it must be that he is tithing with the knowledge of the owner of the pile. However, in that case, by tithing the produce isn’t he benefiting the owner, as he is performing the action in fulfillment of the agency of the owner? Rather, it must be that he separates teruma from his own produce on behalf of the produce of the owner of the pile. The Talmud asks: And with the knowledge and consent of whom is he doing so? If we say it is with the knowledge of the owner of the pile, isn’t he benefiting the owner? He is performing the action in fulfillment of the agency of the owner. Rather, is it not that he is tithing on the basis of his own knowledge, and he is separating from his own produce on behalf of the produce of another? And if you say that in order to tithe from one’s produce on behalf of the produce of the owner, it requires the owner’s knowledge and consent, isn’t he benefiting the owner? Rather, is it not that one may conclude from here that in order to tithe from one’s produce on behalf of the produce of another, one does not require the knowledge and consent of the owner of the pile?