Before us is Mishnah 5 in chapter 10 of Tractate Shevi'is. The basic rule of prozbul is that every loan the lender extended before the prozbul was written is included in it, and therefore he may collect it despite the mitzvah of cancellation of debts in the Shemittah year; but loans extended after the prozbul was written are not included in it. This Mishnah is the source of this halachah, and from it we also learn the difference between a prozbul and promissory notes with regard to predating and postdating.
"Prozbul hamukdam kasher":
A prozbul that was actually written in Adar, while the date recorded in it is the earlier month of Shevat, is valid. The reason: the one who writes the prozbul is the lender, who wants to collect his debt, and predating the document works only to his own disadvantage. Any loan he extended between Shevat and Adar will not be covered by the prozbul and he will not be able to collect it. It turns out that he has diminished only his own rights and has harmed no one else, and therefore he is permitted to do so and the prozbul is valid.
"Vehame'uchar pasul":
But if he recorded in the prozbul a date later than the time it was actually written, it is invalid. For according to the recorded date, loans he extended after the prozbul was written would appear to be included in it, when in truth they are not included, since the prozbul was written before them. And not only is it ineffective for those later loans, but the Sages invalidated it across the board, even regarding the loans that preceded its writing.
Promissory notes and the lien on property:
From here the Mishnah moves on to discuss the status of promissory notes, which are the documents recording a loan. When a person borrows money, his property and land become liened from that moment for the amount of the loan. If the borrower sells some of his land after the loan was made, the lender may collect his debt from that very land that was sold, since his lien preceded the sale. Hence the importance of the precise date: any land the borrower sold after the date of the loan is liened, and the lender can collect it from the buyer.
"Shtarei chov hamukdamin pesulin":
A promissory note in which an earlier date was recorded - the loan was made in Adar, and the note testifies that it was made in Shevat - is invalid. Predating creates the impression that the lien on the borrower's land began already in the previous month, and that the lender has the power to collect even land that the borrower sold before the loan. In truth that land was not liened at the time it was sold, since the loan was made afterward. Therefore the Sages penalized the lender and invalidated the note across the board: it cannot be used to collect at all, not even from land that was sold after the true date of the loan.
"Vehame'ucharin kesherin":
A promissory note in which a date later than the actual time of the loan was recorded is valid, since by doing so the lender worsens only his own position: he deprives himself of the ability to collect from land that was sold before the recorded date. The loss is his own, and there is nothing invalid about it.
How many prozbuls are written for a number of loans?
"Echad loveh mechamishah - kotev prozbul lechol echad ve'echad" - one person who borrowed money from five different lenders: each lender needs his own prozbul for his loans, and therefore five prozbuls are written. A single prozbul of one lender can cover as many loans as he extended.
"Chamishah lovin me'echad - eino kotev ela prozbul echad lechulam" - five people who borrowed from a single lender: one prozbul suffices for all of the loans together.
In summary: in this Mishnah we learned that in a prozbul an earlier date is valid, since the lender thereby worsens only his own position, while a later date is entirely invalid, since it creates the mistaken impression that later loans are included in it. With promissory notes the ruling is the reverse, because of the lien on property: an earlier date is entirely invalid, because it moves up the time of the lien and allows collection from land that was not liened, while a later date is valid, since it causes a loss only to the lender himself. And as for the number of prozbuls: a lender to many people needs only one prozbul for all of them, whereas a borrower from five lenders requires a prozbul for each and every one.