Our Gemara quotes a Mishna in Massekhet Pe’a (3:6), in which Rabbi Akiva taught that even a very small amount of land is obligated in pe’a and bikurim, and could be used as the basis for writing a prosbol.
According to the Torah, among other things the Sabbatical year annulled most private loans (see 15:1-3). Recognizing that lenders were reluctant to offer loans as the Sabbatical year approached – which was, itself forbidden by the Torah (see 15:9-11), Hillel HaZaken established a method that would allow the lenders to collect the debts that were owed to them, even after the Sabbatical year. His suggestion was to write a prosbol that effectively turned the loan over to the courts, which were not constrained by the laws of shemita, since they do not apply to public debts. Thus, when the Sabbatical year was over, the court would be collecting the debt, rather than the individual. This legal fiction was viewed as a benefit for both the rich – who would not be able to recover their loans – and the poor – who would now be able to borrow money when they needed to.
Rashi explains the need for land as the basis for a prosbol as stemming from the fact that this law only applied to “normal” loans. In order to be considered a “normal” loan, land had to be made available as a guarantee that the loan would be repaid.