Kesubos 89a - Pruzbul

Rashi on the mishna explains that pruzbul works based on hefker beis din. In gittin 36a the gemara asks how kesuba works to undermine the d'oraysa hashmata of shemitah, abaye says it is because shemita nowadays is only derabonon, and rava says it is based on hefker beis din. Actually there is a machlokes rashi and tosafos in gittin. Rashi understands rava to be giving an alternative rationale as to how pruzbul works, whereas Tosafos understands that rava is dealing with a side point that the gemara raises over there but we will ultimately have to say that pruzbul is based on shemita being d'rabonon like abaye. Rashi here goes lishitaso by explaining that it is based on hefker beis din like rava. Also, Tosafos explains in gittin that according to rashi that pruzbul is based on hefker beis din, the concept of actual moser shtarosav l'beis din is also based on hefker beis din. This would explain rashi in makos 3b who seems to equate pruzbul with moser shtarosav, since according to rashi they are both based on the same concept of hefker beis din, but according to tosafos the concept of moser shtarosav is based on a pasuk and is not directly related to pruzbul (even though pruzbul borrows the concept).