Sheviis, Chapter 10, Mishnah 6. The Mishnah rules: "Ein kosvin prozbul ela al hakarka" - a prozbul can be written only if the borrower owns land.
Two explanations are given for this:
When the borrower owns land, the loan can be viewed as though it has already been collected by the beis din. The mechanism of the prozbul is that the beis din is regarded as holding the loan, and when the borrower owns land there is something from which to collect, as though the land were already in the hands of the beis din. Accordingly, this is not the collection of a loan, but rather a loan that has to a certain degree already been collected.
It is not common to lend money to someone who owns no land, and the enactment of the Sages was not directed at such an unusual situation. Therefore the borrower is required to own land.
"Im ein lo - mezakehu b'toch sadehu kol shehu":
What is the ruling when the borrower owns no land at all? Even then the lender can take steps that will allow a prozbul to be written: he transfers to the borrower a small plot of land from the lender's own field, and in this way the borrower owns land.
Further extensions of the law:
"Haysah lo sadeh memushkenes ba'ir" - when the borrower owns a field, but it is already mortgaged for another loan, a prozbul may be written on the strength of it. The word "ba'ir" (in the city) is not meant precisely, and it does not mean that the field must be in the same city; it is merely a turn of phrase, and the essence of the law is that a prozbul may be written even on a field that has already been mortgaged.
"V'od: kosvin l'ish al nichsei ishto" - a prozbul may be written for a borrower on the strength of land owned by his wife, even if it is part of her nichsei melog. The reason: with nichsei melog the husband consumes the produce of the field, and therefore he has partial ownership in it, so a prozbul may be written on its strength.
"V'al yesomim - al nichsei apotropin" - when the borrowers are orphans and there is a guardian appointed over them, if the guardian owns land, a prozbul may be written on the strength of his property, even though it is the orphans who owe the money.
In summary: In this Mishnah we learned that writing a prozbul depends on the borrower's owning land, and we examined the two reasons for this. We also learned the ways to make a prozbul valid even when the borrower owns no land of his own: transferring a small plot from the lender's field, a mortgaged field, land owned by his wife, and the property of the guardian when the borrowers are orphans.