Avot315

27:
We are discussing what Rabbi – Rabbi Yehudah the President of the Sanhedrin and compiler of the Mishnah – tried to do with the laws concerning the Sabbatical year, the year of Shemittah. Contrary to my usual wont of keeping the discussion and the explanations separate, in this shiur I depart from my usual custom. I do so because I have received a very apposite comment.

28:
Ed Frankel writes:

I am a bit amazed that the Sages rather than attempting to abolish shemittah did not fabricate a legal fiction that would have allowed farming during shemittah, in the manner that the Prozbul was created so that debts could still be collected, even when shemittah occured.

Before I use Ed's comment to further our discussion I must preface some short explanation. Not only does the Torah [Leviticus 25:1-7] require the land to lie fallow every seventh year but elsewhere [Deuteronomy 15:1-9] it also requires that in that seventh year all monetary debts be cancelled!

(The translation is awkward in one place for a reason which will become clear.)

29:
In the Gemara [Gittin 36a] there is a discussion which derives from this law. The Gemara first quotes a mishnah [Shevi'it 10:3] which reads:

The prosbul was, in fact, a legal fiction. When someone needed credit and it was likely that the incidence of the Sabbatical year would automatically remit the debt it was very difficult for the needy to obtain credit. Thus a law of the Torah which was designed to help the debtor reduce the burden of his debt was now actually increasing it! Hillel instituted that both parties should appear before the Bet Din and sign a document which declared that the debt would be collected on behalf of the creditor by the court, a public body. The word Prosbul itself comes from two Greek words: προς βουλη (pros boulé, which phrase means 'before the court'. (This arrangement assumed that the law only applied to private debts, not to public debts.)

30:
The Gemara now asks an obvious question:

How can Hillel legislate in direct contravention to Torah law expressly stated?

The Babylonian Amora, Abbayé (some 400 years later) explains:

What Abbayyé means is now explained by reference to a baraita:

Rabbi holds that both laws are interdependent, the one is only operative when the other is operative. Rambam, in Mishneh Torah [Shemittah 3:3] explains that the agricultural prohibitions only applied when the Jubilee law applied. The law of Jubilee ceased to be operative with the demise of the northern Kingdom of Israel in 721 BCE. Since the law of Shemittah did not have the force of Torah law in the time of Hillel he was able to institute the Prosbul to alleviate the problem of credit for the needy.

To be continued.