Gemara

36a (Mishnah): A loan for which a (document called) Pruzbul was written is not cancelled in Shemitah. This is one of Hillel's enactments.

Answer #1 (Abaye): The Mishnah is like Rebbi, who says that cancellation of loans in Shemitah is only mid'Rabanan nowadays.

Answer #2 (Rava): Beis Din can make things Hefker.

Question: Did Hillel enact Pruzbul for his Dor (generation), or for all Doros?

If he only enacted for his Dor, any later Chachamim can abolish it. If he enacted for all Doros, later Chachamim can abolish it only if they are greater in Chachmah and number (of Talmidim) than the Chachamim who enacted it.

Answer #1: Shmuel said that a Pruzbul may only be written in the Beis Din of Sura or Nehardai.

If it were enacted for all Doros, one could write a Pruzbul in any Beis Din!

Rejection: Perhaps the enactment for Doros was to write it in a Beis Din like that of Hillel, or of Rav Ami and Rav Asi, which has power to make people's money Hefker. It may not be written in a simple Beis Din.

Answer #2: Shmuel said 'Pruzbul is audacity of the judges. If I had the power, I would abolish it. (Clearly, Shmuel was smaller in Chachmah than Hillel!)

Rejection: Shmuel meant that if he were greater than Hillel, he would abolish it.

(Rav Nachman): (If I had the power), I would perpetuate it.

He meant that he would establish that it applies (loans are not cancelled) even for one who did not write a Pruzbul.

Rabanan of Rav Ashi's Beis Medrash recited the text of Pruzbul in front of each other (without writing it). R. Yonason recited it in front of R. Chiya bar Aba.

(R. Chiya bar Aba): This is sufficient.

(Rav Yehudah): One is believed to say that he wrote a Pruzbul, but it was lost.

This is because once Pruzbul was enacted, a lender can always collect his loan legally. One will not transgress if he can fulfill his desire a permitted way.

Question (Mishnah): A creditor does not collect after Shemitah without a Pruzbul.

Answer: Tana'im argue about this law:

(Beraisa): A creditor must have a Pruzbul to collect (after Shemitah);

Chachamim say, he does not need a Pruzbul.

Rishonim

The Rif cites Shmuel's and Rav Nachman's desires to abolish and perpetuate Pruzbul, and the practice of Rabanan of Bei Rav Ashi (his academy).

Ramban (cited in Ran 19a DH Garsinan): The Halachah does not follow Shmuel, who requires the Beis Din of Sura or Neharde'a. He wanted to abolish Pruzbul! We hold like Rav Nachman, who wanted to strengthen it. The Rabanan of Bei Rav Ashi said the text to each other, and not in front of the greatest Beis Din of the Dor, i.e. Rav Ashi. This shows that even a lesser Beis Din suffices. The Halachah is that two judges suffice for Pruzbul. This is less than what is needed for monetary laws; we do not need a great Beis Din like that of Rav Ami and Rav Asi. Also, if one needs a great Beis Din, why is one believed to say that he wrote a Pruzbul, but lost it? Who says that he will find a great Beis Din, and that they will sign it?

Mordechai (379,380): If the Beis Din of Rav Ami and Rav Asi were required, we could ask them! Rather, the lender can say that the borrower accepted a Beis Din of commoners as if they were the Beis Din of Rav Ami and Rav Asi. We learn from the Yerushalmi that one can say 'I hand over my documents to Beis Din Ploni in city Ploni.' He need not be in front of them.

Ramban (ibid.): We asked from a Mishnah that requires a Pruzbul to collect. We did not say that this is in a place without an esteemed Beis Din, for the Halachah is unlike Shmuel. This is why the Rif omitted Shmuel's teaching, but brought the practice of Bei Rav Ashi. This shows that Hillel enacted for all Doros. If not, later Doros would need a strong Beis Din to forfeit people's money. All the more so, a verbal declaration would not suffice!

Rebuttal (Ran): Even if the Halachah is unlike Shmuel, perhaps Hillel enacted only for his Dor, i.e. until a later Beis Din will annul it. The Gemara said that a later Beis Din can annul it only if Hillel enacted only for his Dor. This shows that annulment is needed, but until then, even a weak Beis Din can write a Pruzbul. The Ramban said 'if it were only for Hillel's Dor, a verbal declaration would not suffice.' Rashi explains that Rabanan of Bei Rav Ashi convened a Beis Din. The only leniency was not to write the Pruzbul. The Rambam permits this for Chachamim who know that Shemitah nowadays is mid'Rabanan, so words can override it. This is why the Rambam brings the practice of Bei Rav Ashi, and also Shmuel's law. Two judges suffice because there is no need to deliberate, but perhaps they must be great Chachamim to forfeit people's money. Surely, the great Beis Din of the Dor must comply with one who requests a Pruzbul. We do not reject a teaching if no one in the Gemara disagrees. The Rambam's opinion is primary. We do not require specifically the Beis Din of Rav Ami and Rav Asi; any esteemed Beis Din in the Dor suffices.

Beis Yosef (CM 67 Sof DH v'Chosvin and Sof DH veha'Rosh): In a Teshuvah, the Rambam does not require specifically the Beis Din of Rav Ami and Rav Asi, but in the Mishnah Torah he requires the greatest Chachamim like the Beis Din of R. Ami and R. Asi, who can forfeit people's money. Other Batei Din cannot. He refers to an esteemed Beis Din in its Dor. Yiftach in his Dor is like Shmuel in his Dor.

Ran (ibid.): The question of whether or not Hillel enacted for all Doros was not settled. However, Shmuel said that he is not strong enough Din to annul it, so all the more so we cannot! The Rambam discusses the enactment Stam, as if it is for all Doros. The Rif cited Shmuel's words that he cannot annul it, to teach that we cannot annul it.

Rosh (4:13): Rav Nachman said that if he could, he would establish that even if one did not write a Pruzbul, it is as if he did, i.e., if he merely said the text in front of judges. The Rabanan of Bei Rav Ashi relied on this. R. Yonason said the text in front of R. Chiya bar Aba. We must say that there were two other judges there, for the text of Pruzbul mentions three judges. R. Tam says that we cannot write a Pruzbul, for we lack a Beis Din like that of Rav Ami and Rav Asi. R. Tam later retracted, and himself wrote a Pruzbul. He said that an esteemed Beis Din of the Dor suffices. Rabanan of Bei Rav Ashi said the text to each other. R. Tam holds that they were considered a Beis Din of experts because Rav Ashi was the greatest of his Dor. I say that the latter Amora'im disagree with Shmuel, who requires the Beis Din of Sura or Neharde'a. The Rif agrees. This is why he omitted Shmuel's teaching, but brought the practice of Bei Rav Ashi.

Rambam (Hilchos Shemitah 9:17): Only a Beis Din of the greatest Chachamim can write a Pruzbul, like the Beis Din of R. Ami and R. Asi. They can forfeit a person's money. Other Batei Din cannot.

Poskim

Shulchan Aruch (CM 67:18): Pruzbul can be written only in an esteemed Beis Din, i.e. three judges who are experts in monetary laws and Pruzbul and Shemitah and many people accepted them on themselves in that city.

Beis Yosef (DH Kosav Ba'al): Sefer ha'Terumos says that such a Beis Din can forfeit people's money.

Rema: Some say that we can write Pruzbul in any Beis Din. It seems that one may be lenient nowadays.

Beis Yosef (DH v'Zeh, citing Teshuvas ha'Rosh 77:6): R. Tam says that even Shmuel requires only a Beis Din powerful in its place. We allow any Beis Din.

Gra (31): The Mechaber (Sa'if 20) holds that the Heter to hand over documents to any Beis Din is only for Talmidim.

Shulchan Aruch (20): If Chachamim lent to each other and said the text of Pruzbul to Talmidim, they need not write it, for they know that Shemitah nowadays is mid'Rabanan, so words can override it.

Rema: Some say that anyone can do so. One can say 'I hand over my documents to Beis Din Ploni in city Ploni.' He need not be in front of them.