According to rashi, abaye and rava argue about the power of hillel to introduce a concept of pruzbul and uproot the mitzvah of shmittah. Abaye holds that it is based on the concept of shmittah nowadays being only d'rabonon (which implies that we pasken like Rebbi, not the Rabbonon), so both the institution and the uprooting are d'rabonon. Rava holds that even according to the Rabbonon that shmittah is d'oraysa, the takana of pruzbul can exist based on the concept of hefker Beis Din. However, when rashi explains the position of Rava, he writes:
דבדבר שבממון אין כאן עקירת דבר מן התורה במקום סייג וגדר דהפקר ב"ד ממון היה הפקר
Rashi is coming to explain the power of the beis din to uproot the hashmata of shivi'is, but why does he mix 2 concepts, one of hefker beis din, and also the concept of geder and siyag that the gemara in yevamos 90b says empowers the chachamim to uproot a mitzvah?
Rashi can be explained by prefacing Tosafos 36a who argues and says that Rava is simply coming to give another answer as to how the chachamim could be mesaken shmittah when it should not exist d'oraysa, but is not coming to offer a suggestion for pruzbul working even within the shittah who holds that shmittah is d'oraysa. The main issue that Tosafos has with rashi is that, even if the Rabbonon had the power to uproot shmittah d'oraysa, they should not do so. With this we can understand why rashi inserts the words "geder" and "siyag" together with hefker beis din. Rashi holds that the power is based on hefker beis din, but the reason that chazal would uproot a d'oraysa mitzvah is becasue of geder and siyag like we find in yevamos that for a siyag they are willing to uproot d'oraysas (but this would not supply them the power to do so because here they are uprooting the mitzvah of shmittah and allowing the lender to collect against the torah, which is not a shev v'al ta'aseh - that is why we also need the hefker beis din rationale).
Being that the end of the shmittah year is mishameit loans, and it is rapidly approaching, I wrote a kuntros this year on the issues that I am making available here.
Please let me know if you find mistakes (either typo or more importantly in content).http://www.jsnseforim.com/pruzbul.pdf