Gemara

(Beraisa): The following apply only when there is Yovel: Eved Ivri, Sedeh Achuzah, and Bayis Ir Chomah;

32b (Beraisa): Yisrael started to count Shemitah and Yovel cycles in the days of Yehoshua, and again in the days of Ezra.

Question: Why did it apply in the days of Ezra? Only a minority of the Shevatim returned!

(Beraisa): Yovel ceased when Reuven was exiled. "U'Kerasem Deror ba'Aretz l'Chol Yoshveha" applies only when all its inhabitants (Bnei Yisrael) are in Eretz Yisrael, but not when (even) a minority (of Shevatim) have been exiled!

"L'Chol Yoshveha" - they must be properly settled, but not if Binyamin dwells in the portion of Yehudah or vice-versa.

Answer (Rav Nachman bar Yitzchak): They counted Yovel cycles in order to be Mekadesh Shemitah in the right time.

This is like Chachamim, who say that Yovel affects the Shemitah cycle. R. Yehudah holds every seventh year is Shemitah, regardless of Yovel. He holds that they did not count Yovelos in the days of Ezra.

Gitin 36a (Mishnah): Hillel enacted Pruzbul...

Question: If mid'Oraisa the loan is cancelled, could Hillel enact that it is not cancelled?!

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

(Beraisa - Rebbi): "V'Zeh Devar ha'Shemitah Shmot" discusses two Shemitos (abstentions), of money, and of land. Shemitas Kesafim applies only when Shemitah of land applies.

Chachamim enacted Shemitas Kesafim (nowadays) for a commemoration of Shemitah. Hillel saw that people were not lending, so he enacted Pruzbul.

Question: If mid'Oraisa the loan is not cancelled (nowadays), could Chachamim enact that it is cancelled?!

Answer #1 (Abaye): Chachamim can tell people to refrain from Mitzvos. (The borrower need not pay his debt.)

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

(Rav Nachman): (If I had the power) I would perpetuate Pruzbul, i.e. establish that one may collect even if he did not write a Pruzbul.

37a (R. Yochanan and Reish Lakish): If there is a document with Acharayus, the loan is not cancelled.

A case occurred, and R. Yochanan ruled that the loan is cancelled. He said 'should we act upon my reasoning'?!

37b (Rav Yehudah): One is believed to say that he wrote a Pruzbul, but it was lost.

Since a lender can collect legally, he will not abandon the permitted way, and collect in a forbidden way.

When lenders came in front of Rav, he would ask 'perhaps you had a Pruzbul, and lost it?'

Rishonim

Rambam (Hilchos Shemitah 9:16): Pruzbul works only nowadays that Shemitah is mid'Rabanan, but not when Shemitah is mid'Oraisa.

Rebuttal (Ra'avad): Abaye says so, but Rava argues and says that Hefker Beis Din Hefker, therefore it applies at all times.

Me'iri (36b DH umi'Kol): Even though this is not a proper argument of Abaye and Rava to apply the rule that the Halachah follows Rava against Abaye, it is proper to rule like Rava. However, I agree with the Rambam. Rava answered only how Chachamim could enact Shemitah. We do not say Hefker Beis Din Hefker to annul a Torah Mitzvah. Beis Din does not want to annul Mitzvos! They enacted only when Shemitah is mid'Rabanan.

Tosfos (36a DH bi'Zman): Rashi says that there was no Shemitah of land in Bayis Sheni. The Yerushalmi says that it applies only when Yovel applies, and there was no Yovel because most Yisre'elim were not in Eretz Yisrael. They counted Yovel in order to be Mekadesh Shemitah, i.e. mid'Rabanan. R. Tam argued, for in Mo'ed Katan (2b) we bring our Drashah (equating Shemitah of land and Shemitas Kesafim) to answer why one may water a field in Shemitah to avoid a loss. It should have brought the Yerushalmi's Drashah! Rather, here Shemitah of land means fields returning in Yovel. Shemitas Kesafim refers to Shevi'is, which is Meshamet (working the land, and also) Kesef. Our Drashah is the Yerushalmi's Drashah!

Rashi (36b DH Rava): Rava teaches that Hillel's enactment is even like Rabanan, who say that Shemitas Kesafim is mid'Oraisa nowadays.

Hasagos ha'Ra'avad (on Rif Gitin 19a): The Rif brought the whole Sugya about Pruzbul. Perhaps he holds that Shemitas Kesafim applies nowadays, or it is merely Midas Chasidus, and it is Meshamet like Rav or Rav Ashi's Talmidim (who said the text of Pruzbul in front of judges). I say that they were all Midas Chasidus. We hold like Rebbi. Nowadays there is no Yovel even in Eretz Yisrael, even mid'Rabanan, for the residents are not there. Even when the residents are there, Teki'as Shofar and freedom of slaves are Me'akev Yovel. The Halachah follows Rav Nachman; one is believed to say that he had a Pruzbul and lost it. Since he acted on this, he surely knew (that Shemitah did not apply in his day). Presumably, Shemitas Kesafim is mere Midas Chasidus, therefore he was lenient. In the days of Rav there was a great Beis Din in Eretz Yisrael, of Rebbi and R. Chiya.

Rebuttal (Sefer ha'Zechus 18a): Clearly, Yovel is only when Yisrael are in Eretz Yisrael, and each Shevet is in its proper place. The only Shevatim that went up with Ezra were Yehudah, Binyamin, and Kohanim and Leviyim who were exiled to Bavel. The land was not divided to the Shevatim; they were mixed together in cities. This shows that Yovel did not apply in Bayis Sheni. We say that they counted Yovels in order to be Mekadesh Shemitah, but Yovel was Batel from when Reuven and Gad were exiled. The one who answered that Yirmeyah returned the 10 tribes agrees that Yovel ceased again after the Churban, until the third Mikdash will be built. They enacted some commemorations of Mitzvos that apply only when Yovel applies, e.g. redemption of houses in a walled city, lest these Mitzvos be forgotten. Surely these did not apply after the Churban, for the land was desolate in the hands of the Yishmaelim. The Yisre'elim there were like guests. Why should they return fields and houses and free slaves?! The Gemara connotes that the other Mitzvos apply only when Yovel applies. I.e. there is no Yovel, they do not apply even mid'Rabanan.

Sefer ha'Zechus: However, Shemitah of land applied mid'Rabanan, like it says in Mo'ed Katan. R. Gamliel and his Beis Din permitted working the land in Erev Shemitah, i.e. but in Shemitah was forbidden. Heterim were given to work the land in Shemitah in order to pay the king's taxes. Many Sugyos show that Shemitah applies to land, therefore it applies also for Kesef everywhere. It says in Kidushin (38b) that Shemitas Kesafim applies everywhere. Even Rebbi agrees, for it says "Ki Kara Shemitah." The Yerushalmi says that according to Rebbi, Shemitas Kesafim is mid'Rabanan nowadays, and Chachamim hold that it is mid'Oraisa. The Bavli connotes that only Rebbi connects Shemitas Kesafim to Shemitah of land. Chachamim hold that Shemitas Kesafim applied even in the Midbar (Kidushin 38b)! The Gemara assumed that Pruzbul applies even though Shemitah is mid'Oraisa. Surely it knew that Yovel was Batel! From the days of Hillel the (grand)son of Rebbi, there was no Beis Din in Eretz Yisrael, so there was no Shofar, returning land or freeing slaves. All of these are Me'akev Yovel! The Ra'avad said that they conducted Shemitas Kesafim for Midas Chasidus. If it was not mid'Oraisa or mid'Rabanan, what was the Midas Chasidus? This discourages loans!

Note: Perhaps it is Midas Chasidus to pardon the borrower, for this is the Torah's desire, even when one need not do so. People will not refrain from lending, since one need not be a Chasid!

Sefer ha'Zechus: One who is not commanded, and fulfills, is called a commoner. Rav would suggest 'perhaps you had a Pruzbul, and lost it. One does not abandon Heter and eat Isur.' I.e. it was an Isur, and not mere Midas Chasidus! R. Yochanan did not want to act on his own reasoning. This implies that it is a real law (not mere Midas Chasidus). It must be mid'Oraisa, for we act based on reasoning for mid'Rabanan laws! Also, Rebbi is an individual. We do not follow him against Chachamim. We hold that Kedushas ha'Aretz of Bayis Sheni is permanent, i.e. even after the Churban. Amora'im were lenient about Pruzbul, for they hold like Rav Nachman, who wanted to perpetuate it, i.e. that any Beis Din suffices. A proof that Shemitas Kesafim applies nowadays is that the Shemitah year was well known in Eretz Yisrael. All the Ge'onim agreed. In Avodah Zarah, the Ra'avad retracted. Also Rashi holds that Shemitas Kesafim applies nowadays.

Ran (on Rif Gitin 20a DH v'Yesh): Shemitas Kesafim is only mid'Rabanan, like Rebbi, since we find that we are very lenient about laws of Pruzbul.

Rashba (Teshuvah 3:32): The Ra'avad wrote in Gitin that Shemitas Kesafim does not apply nowadays, but he retracted in Avodah Zarah (9b DH Ki Zeh) and wrote 'Shemitas Kesafim applies, even though Shemitah of land does not apply. We do not hold like Rebbi. Or, even if we hold like him, Shemitas Kesafim applies mid'Rabanan even in Chutz la'Aretz.'

Tosfos (13a DH Hanach): Even after the 10 tribes were exiled, they continued to count Yovel, for the sake of Shemitah. Why did they cease in Bavel, until Ezra came up? While Yisrael were in Eretz Yisrael, even if Yovel does not apply, Shemitah applies to working the land, therefore they were Mekadesh (Yovel to determine the proper years of Shemitah). In Galus they were not Mekadesh at all. However, they needed to be Mekadesh for the sake of Shemitas Kesafim, which is even in Chutz la'Aretz! If Shemitah does not depend on Yovel, they should (be Mekadesh Yovel in order to) count Shemitah (even when all of Yisrael are in Galus), just like when a minority of are in Eretz Yisrael (and must observe Shemitah of the land), even though we do not count Yovel.

Poskim

Shulchan Aruch (CM 67:1): Shemitas Kesafim applies mid'Oraisa only when Yovel applies. Mid'Rabanan, Shemitas Kesafim applies nowadays everywhere.

SMA (1): The Torah equates Shemitas Kesafim to Yovel. It applies (mid'Oraisa) only when Yovel applies, i.e. when all (i.e. most of) the residents are in Eretz Yisrael.

Gra (5): The Ba'al ha'Ma'or says that we cannot separate Yovel from Shemitah. The Ramban holds that Shemitas Kesafim is mid'Oraisa. This is like Rashi, who says that Rava argues with Abaye about this.