Gemara

(Abaye): If one would despair from something if he knew that he lost it, but he does not know that he lost it, it is not considered as if he despaired;

(Rava): It is as if he despaired.

(Beraisa): One can separate Terumah without the owner's knowledge, and it takes effect. Reuven entered Shimon's field, gathered fruit, and separated Terumah without consulting Shimon. If Shimon does not consider Reuven's separation to be theft, the Terumah takes effect.

If Shimon says 'why didn't you take nicer ones?', and there are nicer fruits, he is sincere, and the Terumah takes effect. If Reuven took the best fruit, the Terumah does not take effect. If Shimon gathered fruit and added to the Terumah that Reuven separated, in either case Reuven's separation takes effect.

Question (against Abaye): When there is better fruit, how does Reuven's separation take effect? When he separated it, he did not know that Shimon would agree to it!

Answer (Rava, on behalf of Abaye): The case is, Shimon had made Reuven a Shali'ach to separate Terumah.

Support: Presumably, this is correct. Had he not made him a Shali'ach, the Terumah could not take effect!

Ameimar, Mar Zutra and Rav Ashi came to Mari bar Isak's orchard. Mari's sharecropper put dates in front of them. Ameimar and Rav Ashi ate, but Mar Zutra did not. Mari found them, and asked his sharecropper 'why didn't you give them from these nice ones?'

Ameimar and Rav Ashi (to Mar Zutra): Why don't you eat now?

(Beraisa): If he says 'why didn't you take nicer ones?', the Terumah takes effect.

Mar Zutra: Rava taught that 'why didn't you take nicer ones?' applies only to Terumah, for people are happy to do Mitzvos (nicely). Here, (perhaps) Mari suggested giving nicer ones only because he was embarrassed.

Kidushin 52b: A brewer was Mekadesh a woman with dregs of beer. The owner found out and said 'why didn't you give her good beer?!'

(Rava): The concept of 'why didn't you take better ones?' was said only about Terumah. Here, the owner said 'why didn't you give better?' because he was embarrassed to say that he does not consent!

Rishonim

Rambam (Hilchos Terumos 4:3): If Reuven separated Terumah (from Shimon's Peros) without permission, or took (stole) Peros from Shimon's field, and separated Terumah, and.Shimon said 'why didn't you take nicer ones?':

If there are nicer fruits, the Terumah takes effect. If there are no nicer fruits, it does not take effect. If Shimon picked more and made it Terumah, in either case what Reuven separated is Terumah.

Beis Yosef (331:31 DH ha'Torem): Also the Tur says so. In Bava Metzia, we say that Shlichus is required! Perhaps 'without permission' in the Tur does not mean that he was not a Shali'ach, rather, he did not have permission to separate the good fruits. However, the Rambam discusses when he did not make him a Shali'ach at all! It seems that he learns from the episode with Mari's sharecropper. Rava taught that 'why didn't you take nicer ones?' applies only to Terumah, for it is a Mitzvah, and the owner wants (to do Mitzvos nicely). A case occurred in which Ploni took Levi's dregs to be Mekadesh a woman. The owner said 'why didn't you give her good beer?!' Rava said that 'why didn't you take better ones?' applies only to Terumah. This implies that in such a case, when he took without any permission, if there were nicer Peros, Terumah takes effect, i.e. even without Shlichus. Since it is a Mitzvah, and he shows now that he consents, it is as if he was satisfied from the beginning and made him a Shali'ach. In Bava Metzia we required Shlichus. That was before we distinguished Mitzvos from other matters. In the conclusion, if he shows that he was pleased about a Mitzvah, it takes effect even if he did not make him a Shali'ach from the beginning. The Rambam cited the Beraisa, and added 'or entered his field' to explain it.

Tosfos (Kidushin 52b DH Im): In Bava Metzia, Reuven had to be a Shali'ach. Since we contrast to there, we must say that also the brewer was a Shali'ach.

Taz (YD 331:15): The Maharshal says that the Tur discusses a Shali'ach who did not have permission to separate the good fruits. The Beis Yosef's defense of the Rambam is wrong. There is no proof from the case of Mari's sharecropper. Shlichus is needed only to make the separation valid retroactively. They asked Mar Zutra why he did not eat after Mari approved! There is no proof from Kidushin, for there Rava ruled, and he holds that Ye'ush she'Lo mi'Da'as is Ye'ush! In Bava Metzia, we say that Rava answered for Abaye. He himself holds that if the owner approves later, the Terumah is valid without Shlichus! No Sugya connotes that Shlichus is not needed according to the Halachah. Tosfos says that the brewer was a Shali'ach, like the Shali'ach for Terumah. The Rambam disagrees. However, how can he say that it is Terumah even if Reuven took for himself (and Shimon approved)? We said that it is not Terumah if there is concern for theft! 'Why didn't you take nicer ones?' applies only to (taking nicer fruits for) Terumah, when it is a Mitzvah, but not to a thief! The Rambam learns from Rava. Rava does not need to say that he was a Shali'ach. Since it is a Mitzvah, it is as if Shimon said from the beginning that he makes him a Shali'ach. We can say that Abaye agrees to this regarding Mitzvos. Rava answered for Abaye that he made a Shali'ach, for he was unsure whether Abaye agrees that Mitzvos are different. There were two questions: one against Abaye regarding Ye'ush she'Lo mi'Da'as, and one even against Rava, that Terumah requires Shlichus. We could have answered both by saying that he made a Shali'ach. In Kidushin, Rava answers differently, that Mitzvos are different. Therefore, we answer like this also for Abaye. To answer for theft, we must say that he made all that he took Terumah. No theft remained; it was entirely a Mitzvah.

Ketzos ha'Choshen (262:1): The Taz says that Reuven made all that he took Terumah, therefore, it is not theft. If Reuven exempted his own Peros, this was not obligatory for Shimon to do. It is a loss for him. In such a case, we do not say that one is pleased to do a Mitzvah (OC 14:4). According to the Shach (358:1), if one picked Peros for himself, there is no theft, for one may do so when he knows that the owner will approve, even if it is not a Mitzvah. We require knowledge only to separate Terumah on the owner's Peros. If he picked to separate for the owner, which is a Mitzvah, the owner need not know. However, this is unlike Tosfos, Hagahos Ashri and the Mordechai. Milchamos Hash-m (Bava Kama 41a DH u'Mah) says that Terumah of a thief takes effect due to Ye'ush and Shinuy ha'Shem. It is not a combination; rather, Ye'ush allows Shinuy ha'Shem to take effect. Without Ye'ush, the Shinuy reverts to like it was initially. Ye'ush she'Lo mi'Da'as suffices, since the Shinuy will not revert. Therefore, if one detached for himself and made it Terumah for his own Peros, he acquires through Shinuy ha'Shem, since (i.e. if) the owner will be pleased when he hears. When he detached for the owner, he cannot acquire through Shinuy ha'Shem (it acquires only for thieves). Tosfos (Sukah 30b DH Ki) says that Ye'ush she'Lo mi'Da'as is only when it came to his hand before the owner knew that it was lost. If it was after, it is proper Ye'ush, even if the owner does not know about the Kinyan (through detaching the myrtle). All the more so, Terumah is a proper Shinuy ha'Shem as long as the owner will not try to undo it.

Ri Korkus: Perhaps when there is a Mitzvah (Terumah), the owner pardons even theft (Reuven detached for himself). Alternatively, Shimon does not consent that Reuven keep what he took for himself. He consents only that the Terumah take effect. However, the Gemara connotes that Shlichus is needed for Terumah. The Rambam says 'he separated Terumah without permission, or took Peros from Shimon's field, and separated Terumah.' These are the same! Perhaps 'separating without permission' is when the Peros were already detached. It is a bigger Chidush that Terumah takes effect even if the Peros were initially attached and not ready for Terumah to be separated. Or, the two cases are separating for himself, and separating for the owner. The Rambam learns from Kidushin that Shlichus is not needed. Alternatively, we challenged Mar Zutra from the Beraisa of Terumah, and did not answer that there, Shlichus is required. Even if Rava requires Shlichus, we rule like Rav Ashi, Ameimar and Mar Zutra. However, perhaps there is no proof from them. They require Shlichus only to make the Terumah valid retroactively, but showing consent now helps from now and onwards. Perhaps in the Rambam's text, the Gemara did not support that Shlichus is needed. Rather, it challenged this, for if so in either case the Terumah should be valid!

Mishneh l'Melech (Bechoros 4:1, b'Sof): Sefer ha'Terumos (188) says that if David asked Ploni to do something, and Levi heard and did it, he is a Shali'ach. Terumah is different, for perhaps David wants only Ploni, who knows the amount and quality that David usually separates. If so, since Ma'aser Rishon and Ma'aser Sheni are fixed (10%), if the entire pile was uniform quality, if another heard a request to tithe and separated, it is valid.

Poskim

Shulchan Aruch (YD 331:31) If Reuven separated Terumah without permission, or took Peros from Shimon's field and separated Terumah, and Shimon said 'why didn't you take nicer ones?', the Terumah takes effect only if there are nicer fruits. If Shimon picked more and made it Terumah, in either case what Reuven separated is Terumah.