According to the Mishna, in a case where a woman demands payment of her ketuba based on the fact that she was divorced, but she could not produce her geṭ, if her husband claimed that he paid it (even though he admits that he cannot produce a receipt for payment), he will not have to pay the money. Similarly, if a man produces a promissory note but cannot produce a prosbol, and the Sabbatical year has passed, the borrower will not have to pay. Rabban Shimon ben Gamliel rules that from the time of the Roman government’s decrees against religious practice, both the wife and the lender would be believed, since it would be dangerous to keep the geṭ or the 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 document – called a prosbol – that effectively turned the loan over to the courts, which were not constrained by the laws of shemitta, 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.
It should be noted that although our Mishna works with the assumption that the lender is not believed to have arranged a prosbol unless he can produce it, whether or not he needs to show it is, in fact, a matter of dispute in Gittin (37b). According to the conclusion of that Gemara we do believe his claim that a prosbol was written.