[Avodah] How does Prozbul work?

[Marty Bluke via Avodah]

R' Zev Sero wrote:
> There's no indication that he mentioned shmita's drabanan status at all.

The simple reading of the Gemara in Gittin 36a-b which is the source of
pruzbul says that pruzbul only works when shemitta is d'rabbanan and in
fact this is how the Rambam understands the Gemara and how he paskens. The
Gemara first states that Hillel was mesaken pruzbul and then asks how could
Hillel be mesaken pruzbul and say that loans are not cancelled when the
Torah says that they are? Abaye answers shemitta nowadays is derabban. The
Gemara then asks how could the Chachamim be mesaken shemittas kesafim
medrabban (and cancel loans) and Rava answers hefker beit din hefker. The
simple reading of the Gemara is that the whole takana of pruzbul only works
when shemitta is derabbanan. Rashi and the Raavad however understand the
Gemara that Rava's answer also answers the first question of how could
Hillel be mesaken pruzbul when the Torah cancels loans based on hefker beis
din hefker and that pruzbul works even when shemmita is d'oraysa.

[Email #2]

R"n Lisa Liel asked:
> I think a better question might be: will prozbul still work when shmitta
> is d'Orayta again?
and R' Zev Sero asked why wouldn't it?

Actually this is an explicit machlokes the Rambam and the Raavad and Rashi
and Tosafos (Gittin 36a-b).

The Rambam writes in Hilchos Shemitta V'Yovel (chapter 9)
    v'ein haprozbul moeel ela b'shmitas kesafim shehee midvrei sofrim
    aval shemitta shel torah ayn haprozbul moeel bo

The Raavad there argues that prozbul will work even when shemmita is
d'oraysa based on his understanding of Rava's answer of hefker beis din
hefker (like Rashi in Gittin).