Sheviis 10:3

A debt supported by a prozbul is not canceled. Hillel instituted the prozbul when he saw that people were refraining from extending loans out of fear of them being canceled. This violated the Torah’s warning not to refrain from loans in this fashion (Deut. 5:9).

Sheviis 10:4

A prozbul is a document that transfers debts to the local judges so that they might be collected. It must be signed by the judges or by witnesses.