Gemara

Rav Ashi sent 17 Zuz to Rav Acha brei d'Ravina (a Kohen) for Pidyon ha'Ben, and asked him to return the extra third of a Zuz.

Nedarim 72b (Beraisa - R. Yonason): In the entire Torah, Reuven's Shali'ach is like himself (the Shali'ach's actions are attributed to Reuven).

Eruvin 81b (Mishnah): Zachin l'Adam she'Lo b'Fanav (we may do something beneficial for a person in his absence) but we cannot do something detrimental to him in his absence.

Kidushin 29a (Beraisa): If a man must redeem himself and his son, he first redeems himself;

R. Yehudah says, he first redeems his son, for it is his own Mitzvah to redeem himself, but his son's redemption is the son's Mitzvah placed on the father!

(R. Yirmeyah): If the father only has five Sela'im, all agree that he redeems himself. A Mitzvah of his own body takes precedence. (They argue about collecting the money from sold property.)

Rishonim

Ran (Pesachim 3b DH v'Kashya): Even though Pidyon ha'Ben is incumbent on a person, he can exempt himself through another. When someone else does such a Mitzvah, all agree that he blesses 'Al'. Amora'im argue about when the person does them himself.

Question (in Rivash 131): Why does the Rambam say to bless 'Al Pidyon ha'Ben' when redeeming his son? The Mitzvah is on the father. We expound that if one was not redeemed, he has a Mitzvah to redeem himself. There is no Mitzvah through a Shali'ach or through Beis Din...

Poskim

Rema (YD 305:10): The father cannot redeem through a Shali'ach. Beis Din cannot redeem the son without the father.

Taz (11): The questioner in the Rivash said so, and the Rivash did not mention this. The Rema inferred that the Rivash agreed. This is astounding. In the entire Torah, one's Shali'ach is like himself. What is the source to exclude Pidyon ha'Ben?! Tzedah l'Derech (Parshas Bo) asked this. He concluded that if the father died before 30 days, the grandfather or Beis Din can redeem. He brought proofs, and testified that the Maharal of Prague ruled like this. He is correct. It is clear from the Ran that a Shali'ach can redeem a son.

Drishah (3 DH Im): The Rivash connotes that the father cannot make a Shali'ach for Pidyon ha'Ben, and Beis Din cannot redeem. Semak holds that even in another city (the son is not there), a Shali'ach can redeem.

Shach (11): In the entire Torah, one's Shali'ach is like himself. The Ran says that one can redeem through a Shali'ach. Ma'adanei Melech (Sof 266) was astounded at the Rema. Tzedah l'Derech says that one can redeem through a Shali'ach. Several Gedolei ha'Dor agreed. He said that even the Rivash said only that the Torah did not obligate others to be a Shali'ach for the father,

Gra (17): The Rema is astounding. Not only can one redeem through a Shali'ach, for one's Shali'ach is like himself. Even one who is not a Shali'ach can redeem. This is no more stringent than Ma'aser Sheni (which anyone can redeem). We need not say that the Rivash agreed to the questioner.

R. Yom Tov Algazi (62b, 79): We find that Rav Ashi sent the money for Pidyon ha'Ben through a Shali'ach. The primary Mitzvah is giving the money to a Kohen. This shows that it can be done through a Shali'ach, unlike the Rema. Do not say that he already did the Mitzvah beforehand, but since he did not have coins, he gave a security, and later sent money to redeem the security. Giving a security does not redeem.

R. Shimon Satin (the questioner in Chasam Sofer 2:293): All the Acharonim argue with the Rema. I defend him. The Rivash himself holds that one should not redeem through a Shali'ach. He says that we bless 'Al', and not 'Lipfdos', because another party is involved, i.e. the Kohen. The father does not do it by himself. However, the Rambam says that one who redeems himself blesses Lifdos! Rather, the father can do the Mitzvah through the son when the son matures. Then, he can do it only himself. (Note: I do not understand this. If the text said 'until then, he can do it only himself' I would understand it.) If one must redeem himself and his son, he redeems himself first (Kidushin 29b). This shows that the Mitzvah is primarily on himself, just a minor cannot do it himself. The Rashba (2:321) says that also when the son matures, the father has first right to redeem, but if the son redeemed himself first, he merited. Keneses ha'Gedolah says that the Rivash disagrees. I say that the primary Chiyuv is on the son, and not on the father. Since a minor cannot redeem himself, the Torah made the father a Shali'ach. Therefore, when the son matures, the father's Shelichus ceases. This is like a Na'arah (a girl in the first six months after Bas Mitzvah). She or her father can accept Kidushin. The Rishonim say that the Torah made her a Shali'ach of her father, against his will. This is why she cannot make a Shali'ach (to accept Kidushin). Similarly, the Torah made a father a Shali'ach for his Bechor, b'Al Korcho, so the father cannot make a Shali'ach. One who redeems himself can make a Shali'ach, like in the entire Torah.

Rebuttal (Chasam Sofer): We find a Shali'ach b'Al Korcho of the Meshale'ach, but we do not find that b'Al Korcho, one is forced to be a Shali'ach (i.e. the father is obligated to redeem his son)! However, we find this regarding Kohanim. The Gemara was unsure if they are Sheluchim of Yisrael. This is b'Al Korcham. If a Kohen abandons the Avodah and leaves, he is Chayav Misah! Perhaps similarly, a father is his son's Shali'ach b'Al Korcho. However, how can a Shali'ach do what the Meshale'ach cannot? Also, why is the lien on the father's property, even if the son has property? The Rashba and Rivash argue about whose Pidyon has precedence (one's own, or his son's), like Rashi and Tosfos (Kidushin 29b). We need not say that the father is a Shali'ach for his son. The custom is not to redeem through a Shali'ach, more than for other Mitzvos, since Pidyon ha'Ben commemorates Hash-m's redemption of Yisrael from Mitzrayim, which was through "Ani" (Hash-m,) and not a Shali'ach.

Chasam Sofer (YD 297): Shelichus of sending the father's money to a Kohen is like sending it on a monkey. He informs the Kohen that it is for Pidyon ha'Ben, and he is Yotzei. No one would say that a Shali'ach does not work for this. (We needed sources for Shelichus for Gitin, for the Get must come from the husband's hand to her, and for Kidushin, in which the husband must make a declaration.) Poskim argue about a Shali'ach who gives his own money, without acquiring it for the father. The questioner in the Rivash was sure that this does not work. Many erred greatly about this.

Pischei Teshuvah (16): Be'er Heitev says that the Acharonim say that one can redeem through a Shali'ach. One can redeem on Tanai, and Tanai works only for matters to which Shelichus applies (YD 234:2). Chemdas Shlomo (32) explains like the Chasam Sofer, and says that the Rema's opinion is primary. Dagul me'Revavah allows making a Shali'ach after 30 days, but it is improper within 30 days (before the Mitzvah applies). However, even if the Shelichus does not take effect, the Shali'ach can redeem (after 30 days) due to Zachin l'Adam, since we know that the father wants this! Pnei Arye says that a Kohen or Levi (or perhaps even their wives) cannot be a Shali'ach l'Chatchilah in place of the father, since he himself is exempt. B'Di'eved, perhaps he need not redeem again. The Chasam Sofer (2:296) suggested that surely a Kohen can be a Shali'ach, since we redeem with him, just like a man or woman can be a Shali'ach to give or receive a Get (even though anyone can do only one of these for himself), but perhaps a Levi is Pasul. However, perhaps a Kohen disgraces his Kehunah if he is a Shali'ach, just like a Kohen should not read the words for other Kohanim giving Birkas Kohanim. He concluded that a Tzibur of Leviyim must redeem Bechorei Yisrael (who were not redeemed), so surely they can be Sheluchim.

Pischei Teshuvah (16): The Gaon of Lisa says that we can redeem a minor from his own money. It is unlike other Mitzvos, for here there is a Shibud. Here, it is not due to Arvus (Yisre'elim are guarantors for each other). Rather, the Kohen has a share in the property. If he seizes, it helps. All the more so, Beis Din collects from his property. He says that we cannot stipulate that the minor's money should be a gift (if the minor will not redeem himself later), and it does not help with others' money, unless we are Mezakeh them to the minor on condition to redeem with them.

Rebuttal (Chazon Ish YD 185 DH Kidushin): There is a Shibud also for Machatzis ha'Shekel, yet Reuven may give a (half-)Shekel for Shimon! He need not be Zocheh the money to him, for we permit this even when Shimon is Mudar Hana'ah from Reuven (Nedarim 33a)! Rather, we learn from Eved Kena'ani (a stranger may pay money to redeem him) that one may pay another's debts. The same applies to Pidyon ha'Ben. However, the Chasam Sofer connotes that the Rema says that Pidyon ha'Ben cannot be through a Shali'ach only when the Shali'ach gives his own money. Even if a Shali'ach gives his money, the father redeems, unless he tells the Shali'ach to redeem. Then, the Shali'ach does the Mitzvah, whether it is his money or the father's. The Acharonim agree that we can redeem without the father's Da'as, for it is Zechus. However, a minor orphan is not liable, so perhaps it is better that he do it later himself. We cannot use his money now, for he is exempt now. This is like the Taz. The Gra did not reveal his opinion about an orphan.

Chasam Sofer: If the father gives five Sela'im to someone who is Zocheh for the Kohen, he was Yotzei. This is unlike Pidyon Peter Chamor, for there, the Yisrael has no Acharayus (responsibility for any loss). Here, the father has Acharayus until it reaches the Kohen or one who is Zocheh for him. Then, he did his Mitzvah. The one who was Zocheh is liable if he was negligent, like any Shomer. If he cannot pay, and the father was negligent for giving to him, he is liable for negligence with another's money, but not due to Mitzvas Pidyon ha'Ben.