Sheviis 10:5

A prozbul is valid if predated but not if it is postdated. A document of debt is valid if postdated but not if predated. If there’s one borrower and five lenders, five prozbuls are needed. If there’s one lender and five borrowers, a single prozbul covers them all. (This is because the prozbul pertains to the lender.)

Sheviis 10:6

A prozbul may only be written against land. If the borrower has no land, the lender grants him a tiny share in his own field. If the borrower has mortgaged land in the city, a prozbul may be written against it. Rabbi Chutzpis says that a prozbul may be written against property belonging to the borrower’s wife, or for orphans against property belonging to their guardian.