Hillel would have had no authority to contravene a biblical law, but since shemittat kesafim no longer is a biblical precept, he invoked the principle of (Psalms 119:126), “Et la’asot LaShem, heferu Toratecha – It is a time to act for G-d: they have voided Your Torah.” Commenting on the mishnah in Berachot 54a that cites this verse, Rashi explains that at times we void Torah laws on behalf of G-d in order to uphold His Torah.
The word Prosbul is Greek. R. Chisda (Gittin 36a-37b) said it is a compound word – pros-buli-u’buti. Pros means “the order of the matter.” Buli refers to the rich (see Rashi ad loc.), and buti means the poor. The Prosbul was created for the sake of the rich as well as the poor. With it, the rich can collect their money after shemittah and the poor are able to obtain loans before it.
(To be continued)