Gemara

(Beraisa #1): Pidyon ha'Ben applies after 30 days. Pidyon Peter Chamor applies immediately.

Contradiction (Beraisa #2): At least 30 days are required for Pidyon ha'Ben and Pidyon Peter Chamor.

Resolution #1 (Rav Nachman): Beraisa #1 teaches that if Pidyon Peter Chamor was done immediately (i.e. before 30 days), it is valid (b'Di'eved).

Question: This implies that Pidyon ha'Ben is invalid even b'Di'eved within 30 days. Rav was Machshir such a Pidyon!

Answer (Rava): If he said that the Pidyon should take effect immediately, all agree that it is invalid. (Rav is Machshir when he gave the money now to take effect after 30 days.)

Resolution #2 (Rav Sheshes): Beraisa #2 teaches that one does not transgress (for delaying the Pidyon) until 30 days.

49a (Mishnah): If Reuven's Bechor died on day 30, this is like dying within 30 days. (If Reuven already gave five Shekalim to a Kohen, the money is returned.)

R. Akiva says, if he already gave, it is not returned. If he did not yet give, he need not give.

Chachamim learn a Gezeirah Shavah "Chodesh-Chodesh" from the counting of the Bechoros in the Midbar. Just like only Bechoros above 30 days were counted, also Pidyon ha'Ben applies only after 30 days.

R. Akiva is unsure. It says "va'Ma'alah" regarding Erchin to teach that a 30-day old is like less than 30 days (he has no Erech). We do not learn this from Bechoros (in the Midbar). Therefore, Erchin and Bechoros are Shnei Kesuvim. We do not learn from them to other places. However, perhaps Pidyon ha'Ben is considered the same matter!

(Rav): If Pidyon ha'Ben was done within 30 days, it is valid;

(Shmuel): It is invalid.

If he said that the Pidyon should take effect immediately, all agree that it is invalid. If he gave the money now to take effect after 30 days, and the money is intact (i.e. the Kohen still has it), all agree that it is valid. They argue when he gave the money now to take effect after 30 days, and the Kohen does not have the money then. Rav says that it is valid, just like Kidushin (money given to take effect after 30 days);

Shmuel says that Kidushin is different, for he could be Mekadesh her right now, but Pidyon cannot be done within 30 days.

Question (Mishnah): If Reuven's Bechor died within 30 days, even if he already gave five Shekalim, the money is returned.

Inference: Had the Bechor not died, the Pidyon would be valid, even though it was done within 30 days!

Answer: The case is, the Kohen still has the money after 30 days.

(A reciter of Beraisos): If Pidyon ha'Ben was done within 30 days, it is valid.

Objection (Rav Yehudah): That is wrong, for it is unlike Shmuel!

Even though normally the Halachah follows Rav in Isurim, here the Halachah follows Shmuel.

Rishonim

Rambam (Hilchos Bechoros 11:18): If one redeemed his son within 30 days, if he said 'from now', he is not redeemed. If he said 'after 30 days', he is redeemed even if the money was consumed within 30 days.

Rosh (8:5): A Beraisa said that the son is redeemed. It discusses when the money was consumed, for Rav Yehudah rejected the Beraisa, since it is unlike Shmuel. We follow the rule that the Halachah follows Rav in Isurim, even though Rav Yehudah holds unlike this rule and rules like his Rebbi Shmuel. However, why did he reject the Beraisa just because it is like Rav? Rather, the Beraisa did not specify whether the Pidyon was now or after 30 days. Rav Yehudah said that Shmuel holds that he is not redeemed even after 30 days if the money was consumed, and also Rav says so if he redeemed 'from now.' Therefore, it is better if the Beraisa says Stam 'he is not redeemed', so it is like everyone. It is like Rav if he said 'from now', and it is like Shmuel even if he said 'after 30 days', if the money was consumed. In this way, people will not err to be lenient. Bahag says Stam 'if he redeemed within 30 days, he is not redeemed.' We must say that this is when he said 'from now.'

Terumas ha'Deshen (269): If day 31 will fall on Shabbos, we do not seek ways to redeem on Shabbos, even though it is possible to redeem with a Kli that is permitted to move on Shabbos, or to give money before Shabbos and stipulate that the redemption take effect after Shabbos, like the conclusion in Bechoros. Rav and Shmuel agree that if he stipulated that the Pidyon take effect afterwards and the money in intact, he is redeemed. One should not redeem with a Kli on Shabbos, for this is like business. This is why we do not make Hekdesh on Shabbos. To redeem before and stipulate is difficult. How can one bless or make the customary Seudah, which some say is a Seudas Mitzvah?! Even though the Rambam, Semag, Semak and Rosh rule like Rav, Tosfos (Bava Kama 70a DH a'Metaltelim) and Or Zaru'a rule like Shmuel. Their text says that the Halachah follows Shmuel; it is primary. Even the Rosh in Nidah (1:1) says 'the Halachah follows Rav in Isurim, like it says in Perek Yesh Bechor', and here is the only place in the Perek where it says so! Since we hold like Shmuel, his son is not redeemed yet, lest the money be consumed.

Poskim

Shulchan Aruch (YD 305:11): A Bechor cannot be redeemed until 30 days passed. After 30 days he is redeemed immediately, not to delay the Mitzvah. If the 31st is on Shabbos, we do not redeem him on Shabbos. Rather, we wait until day 31.

Maharshal (Teshuvah 7): If a baby was redeemed on day 29, do we redeem it again on day 31 with Berachos? There are different texts in Bechoros. The Rosh's text does not say that the Halachah follows Shmuel. Therefore, he says that the Pidyon is valid after 30 days even if the money was consumed. It seems that this is even if he redeemed Stam (without specifying that it take effect after 30 days). If the Mishnah discusses one who stipulated (but did not explicitly say so), what was the question against Shmuel? We can say that it discusses when the money is intact! Rather, the Mishnah discusses Stam. Rav holds that even though the money was consumed, he is redeemed. They argue about one who stipulated (even though they could have argued about Stam) to teach the extremity of Shmuel's opinion. Maharik proved that the Mishnah discusses Stam, for if not, what is the Chidush that if the baby died within 30 days, the money is returned? There was no Pidyon, for he stipulated that it is only after 30 days, and the baby died beforehand!

Maharshal: Bahag says Stam 'if he redeemed within 30 days, he is not redeemed.' The Rosh said that this must be when he said 'from now.' The Rosh did not want to say that this discusses Stam. This shows that the Rosh holds that Stam is like specifying 'after 30 days'. Even though not everyone knows Halachos, he wants to redeem his son properly, whenever is proper. However, the Tur wrote only 'if he said 'after 30 days', he is redeemed even if the money was consumed.' This connotes Stam, he is not redeemed. Also, Semag, Semak and the Ramban teach that he is redeemed even if the money was consumed, only if he said 'after 30 days.' We must say that the Rosh agrees. Stam helps only if the money is intact. Even though the questioner thought that Stam helps in every case, even if the money was consumed, after we answered that the money in intact, even Rav agrees. Rav and Shmuel argue only about one who specified. Bahag did not discuss Stam, for then he is redeemed if the money in intact. Therefore, he discusses 'from now.' Then, he is not redeemed at all. He does not hold that Stam does not help even if the money in intact, for I proved that the Mishnah discusses Stam. Also, the Rosh would have established Bahag to say that he is redeemed in this case.

Maharshal: Perhaps the Rosh's text said somewhere in this Perek 'the Halachah follows Rav in Isurim'!

Note: If in the Rosh's text this was taught here, he would not have needed to prove that we rely on this against Rav Yehudah!

Maharshal: The Semag's text said that the Halachah follows Shmuel, and even so he says that if he stipulated 'after 30 days', or even Stam and the money was not consumed, he is redeemed! Maharik answered that Reuven did not say 'from now' or 'after 30 days.' Rather, the Gemara teaches when the son is or is not redeemed. Semag holds that they argue about Stam, and also when he said 'after 30 days' and the money was consumed. The Gemara rules like Shmuel regarding Stam. 'After 30 days' is like Kidushin.

Bach (14): If day 31 is on Shabbos, if the baby was born at the beginning of Wednesday night, he is already a month old two or three hours into the day on Friday morning, so we redeem him on Friday and bless and make the Seudah, for his time already came. This is the ideal Mitzvah. He does not transgress the primary time to redeem at all. Rav Sheshes taught that if one does not redeem after 30 days, he transgresses! However, if he was born at the end of the day on Thursday, he is not a month old until Shabbos. One should not redeem him on Friday and stipulate that it takes effect after 30 days. Since now it is not time to redeem, the Berachos are Levatalah. Also, there is no reason for a Seudah. This is unlike the Maharshal. Terumas ha'Deshen, the Maharil and the Shulchan Aruch say that (in this case) we redeem on day 32. I say that he should be redeemed on Friday if 29 days (and 12 hours...) have passed. If not, we redeem him on Sunday. The Yere'im rules like this.

Shach (12): If day 31 is a fast day, if 29 days (and 12 hours...) passed on day 30, we redeem then. If not, we redeem the night after the 31st. We do not redeem during the day and make the Seudah at night, for this does not publicize the Pidyon. The Terumas ha'Deshen explained that the Seudah publicizes only if it is when the Pidyon takes effect. However, distinguish a Ta'anis from Shabbos, and say to redeem during the day. The Terumas ha'Deshen said that one cannot bless on Erev Shabbos, for he is not yet redeemed, nor on Shabbos, for then the person does nothing and he is redeemed automatically. We must bless on all Mitzvos beforehand. If he did not, he does not bless afterwards.

R. Akiva Eiger (DH v'La'asos): We do not redeem and make the Seudah the night before the 31st, for sometimes 29 days (and 12 hours...) have not passed. If the Pidyon was postponed to the 32nd (e.g. the 31st was Shabbos) and the 32nd is a Ta'anis, we redeem and make the Seudah the night before.