As we've seen previously in this series, Israel was sent into exile for failing to follow the Shemitah/Jubilee laws. Did the ruling class of Israel learn its lesson after having been sent back to Israel? Having been forgiven their debt (their obligation to God to honor these rules), did they turn around and forgive the debtors, the poor in their economy? No, like the Ungrateful Servant in Jesus' parable, they circumvented the law and insisted on being paid.
According to the Talmudic scholar/Christian convert, Alfred Edersheim's comprehensive book, The Temple, not long before the time of Jesus, scribes and lawyers created a workaround:
"Rabbi Hillel…devised a formula called 'prosbul' (probably 'addition,' from a Greek word to the same effect), by which the rights of a creditor were fully secured. The 'Prosbul', ran thus : 'I, A. B., hand to you, the judges of C. D. (a declaration), to the effect that I may claim any debt due to time at whatever me I please.' This 'Prosbul,' signed by the judges or by witnesses, enabled a creditor to claim money lent even in the sabbatical year. (Mish Shev, sct x.) and though professedly applying only to debts on real property, was so worded as to cover every case.""
(Edersheim, pg. 191-192
Nicholas Perrin, in Jesus the Temple, makes the same point:
"According to the Mishnah (Sheb. 10:3) the prosbul was enacted by Hillel (first century BCE) to counter the provision in the Torah for cancellation of debts every seven years (Deut 15:1-18; also Exod 21:2-6; 23:10-11; Lev 25:2-7). It was a legal fiction that assigned the loan to the court to collect a debt…
("A prosbul-loan is not cancelled [by the seventh year] (m. Sheb. 10:3)…
The point is also documented by Professor David Fiensy in his excellent Christian Origins and the Ancient Economy:
"The papyrus text from Wadi Muraba'at… dating from the year 56 CE, may illustrate the application of the prosbul. Goodman maintains that the prosbul would have been necessary only for rich people lending to poor ones. Rich persons lending to other rich persons could count on repayment because of the pressures of social stigma and thus would not need special legislation. Goodman assumes that many peasants would have been unable to repay the loans and thus would have faced foreclosure on their farms.""
Everything that I've written above presents a rich background against which to understand the teachings of Jesus which pertain to debt, debt forgiveness, and the consequences of neglecting the latter.