Gemara

150b (Shmuel): On Shabbos, one may say 'I will go to city Ploni' (outside the Techum), because if there were shacks [every 70 Amos along the way to extend the city], it would be within the Techum.

Question (Mishnah): One may not be Machshich (go to the end of the Techum towards the end of Shabbos) to hire workers or to bring Peros.

One could bring Peros if there were Mechitzos! (Shmuel should permit this.)

Answer: The case is, the fruits are attached.

Question (Mishnah): One may be Machshich for the needs of a Kalah or a Mes (i.e. shrouds).

One may be Machshich for similar needs (i.e. clothing) even of a live person [since it would be permitted if there were Mechitzos]!

Answer: One may be Machshich to fix shrouds.

(Mishnah - Aba Sha'ul): The general rule is [if one may discuss doing something, he may be Machshich for it].

151a: He discusses the following law:

(Shmuel): Reuven may tell Shimon 'guard my Peros in your Techum, and I will guard your Peros in my Techum.'

Aba Sha'ul says to the first Tana 'you agree that one may say 'guard my Peros in your Techum...' You should teach 'whatever one may say, he may be Machshich for it'!

The general rule includes the following law;

(Beraisa): One may be Machshich to attend to the needs of a bride or a Mes, to bring a coffin or shrouds. One may say [to a Shali'ach] 'go to place Ploni to buy...'

Rishonim

Rashba (151a DH Amar): Reuven may tell Shimon 'guard my Peros in your Techum, and I will guard your Peros in my Techum', even though Reuven may not go to where his Peros are. Since it is permitted for Shimon, there is no Isur to say this. Tosfos derives that if Levi accepted Shabbos before dark, he may tell David to do a particular Melachah for him, since it is permitted for David.

Ran (64b DH umid'Amrinan): This is not a proof. Here, if there were shacks, Reuven himself would be able to guard them!

Beis Yosef (OC 263 DH Kasav ha'Ran): I do not understand the Ran. We can also say that had Levi not accepted Shabbos, he could have done Melachah!

Taz (3): This is no rebuttal. The reasoning "there is a Heter without this" helps only if there is a Heter now, but not retroactively, i.e. had he done differently, it would be permitted now. Now, there is no Heter! If not, why did Shmuel permit saying 'I will go to city Ploni' because 'if there were shacks...'? He should say that he could have gone close to there before Shabbos! Rather, the reasoning helps only for what one could do now. If one accepted Shabbos Stam, he accepted it for everything.

Taz (3): I say that the Ran's question is not difficult because the Gemara shows that the Heter to ask him to guard is not due to shacks. Shmuel taught the Heter of shacks regarding 'I will go to city Ploni tomorrow.' The Gemara asked from the Mishnah that forbids being Machshich to bring Peros. This implies that if not for Shmuel's teaching, we understood the Mishnah, even though the Seifa permits being Machshich to guard Peros. We must say like Rashi, that guarding Peros is intrinsically permitted, if he were in their Techum. This shows that the Heter of the Mishnah is not due to shacks, rather, because there is no action of Isur in guarding.

Taz: Do not say that after Shmuel's teaching, we understand the Mishnah differently, that it is due to shacks. If so, he should have explained the Mishnah, and not teach a new case. Also, if the Heter is due to shacks, why did Shmuel need to teach a second teaching about 'I will go to city Ploni'? I explain that we explain Aba Sha'ul's general rule according to Shmuel. He tells the first Tana 'you should teach a general rule', which will teach about needs of a Mes or Kalah. Shmuel teaches that the Mishnah is not due to shacks [that permit bringing ready shrouds]. Rather, it is to cut shrouds. The Rashba properly learned that the Heter to guard is not due to shacks, rather, because it is not an absolute Isur. Similarly, if one accepted Shabbos early, he may tell others, for one may command about anything not forbidden to everyone. The Ran did not argue with the law, just he rejected the proof.

Poskim

Shulchan Aruch (OC 263:17): If Levi accepted Shabbos before dark, he may tell David to do a particular Melachah for him.

Magen Avraham: He did not need to accept Shabbos. Whatever has a Heter, one may say it (307:8).

Mishnah Berurah (64): If it is close to Bein ha'Shemashos, surely most of the Tzibur accepted Shabbos, and the minority are drawn after them against their will (Sa'if 12, so everything is forbidden).

Rema: One may benefit from the Melachah on Shabbos, and all the more so on Motza'ei Shabbos.

Magen Avraham (31) and Gra (DH v'Chol): All the more so it is permitted on Motza'ei Shabbos, for he can end Shabbos whenever he wants.

Rema (ibid.): If one delays to pray on Motza'ei Shabbos, or his meal continued into the night, he may tell one who prayed and was Mavdil to do Melachah for him, e.g. to light Neros or cook for him. He may benefit from and eat the result of the Melachah.

Magen Avraham (32): The Rema discusses one who prayed and was Mavdil in Tefilah (said Atah Chonantanu).

Levush (17): The Rema holds that even if one accepted to add to Kedushas Shabbos, this means that he will not do Melachah. He did not forbid others' Melachah to himself. I distinguish Motza'ei Shabbos from Erev Shabbos. Erev Shabbos depends on his vow and acceptance. We can say that Levi accepted Shabbos, and David did not. At the end of Shabbos, one continues the Kedushah of Shabbos. All the Isurim continue. We cannot say that some Isurim were permitted, and others were not. Since he may not do Melachah, also what others do, whether Nochrim or Yisre'elim, is forbidden to him until he blesses Birkas ha'Mazon and diverts his mind from Shabbos. Until he blesses and says Retzei in Birkas ha'Mazon, he does not divert his mind from Shabbos. I learn from the Tur (624). Some Chasidim used to observe two days of Yom Kipur. If there were 10, they prayed the entire Yom Kipur Tefilah with a Minyan. The Rosh protested against the Tefilah, but all Isurim of Melachah are like the first day. Therefore, others may not cook for such a person. The Beis Yosef brought that Hagahos Maimoniyos (after Hilchos Shevisas Asor) and Maharam (76) forbid what others cook for him [even if the second day is Erev Shabbos, so he cannot cook for Shabbos]. He may not be Makneh his flour to them for them to cook it. Since he extends Yom Kipur, he may not eat what others cook for him. The same applies here. However, they permit him to eat what others cooked for themselves, as long as they do not cook extra for him. However, if one blessed Birkas ha'Mazon but did not yet say Havdalah or pray, he may ask others to do Melachah, since he diverted his mind from Shabbos. Perhaps the Rema discusses such a person, but his words connotes unlike this.

Taz: The Rema is correct. The Levush thought that the Rashba's Heter is because he did not accept to forbid others' Melachah to himself. This is wrong. If so, he need not learn from bringing Peros, which discusses on Shabbos itself!

Magen Avraham (33): This is unlike people who observe two days of Yom Kipur. At night [after the second day], they may not eat what others cooked during the day, for they hold that it is Asur for all of Yisrael [to do Melachah on the second day]. Here, he knows that Shabbos ended, just he was not Mavdil. I do not understand the Levush. Why should Melachah depend on Birkas ha'Mazon? If he said [Baruch] ha'Mavdil Bein Kodesh l'Chol, he may do anything. It is unclear whether he may say Retzei afterwards in Birkas ha'Mazon.

Machatzis ha'Shekel: According to the Beis Yosef and Magen Avraham, if most of Levi's Beis ha'Keneses accepted Shabbos, he may not command someone from another Beis ha'Keneses. The Bach and Taz permit, if the other Beis ha'Keneses did not yet accept Shabbos.

Mishnah Berurah (67): The Acharonim hold like the Shulchan Aruch (i.e. Rema), unlike the Levush. After Shabbos, one may immediately be Mavdil and do Melachah, even though most of the Tzibur did not.

Kaf ha'Chayim (102): Many rule like the Rema. The Bach says that one who is stringent like the Ran will be blessed. If someone else made food by himself, all permit one who is extending his meal to eat from it.

Mishnah Berurah (673:27): If one accepted Shabbos and his Ner [Chanukah] extinguished, if it is still before Shabbos, he may tell a friend to relight it for him.

Yom Tov Sheni k'Hilchasah (14:2 and Ha'arah 3): On Yom Tov Sheni, a Ben (resident of) Chutz la'Aretz [visiting Eretz Yisrael] may not tell, or even motion, to a Ben Eretz Yisrael to do Melachah for him. I.e. he may not point to a light, to hint that he should light it. He may say 'it is dark in here', for this is unlike Shelichus (from ha'Gaon R. S. Z. Auerbach, Ztz"l).