Bava Metzia, Chapter 1, Mishnah 6. From here on, the Mishnah, and indeed the rest of the entire chapter, moves on to discuss the laws of found documents: to whom should the document be returned? The documents our Mishnah deals with are promissory notes, loan documents, which state that Reuven borrowed money from Shimon - which means Shimon lent money to Reuven.
The Nature of a Promissory Note:
Generally, the lender, Shimon, keeps the loan document in his possession, as this is his proof that Reuven owes him a hundred dollars. If Reuven does not pay, Shimon takes the document to beis din and claims that this paper is worth a hundred dollars, and beis din enforces the collection. Once the debt is paid, the lender hands the document over to the borrower so that he cannot collect a second time, and the borrower tears or destroys it so that they cannot claim the debt from him again.
Logically, it would make sense that someone who finds a promissory note should return it to the lender, who is the most likely person to be its owner. However, returning it could harm not only the borrower or the lender, but even a third party - the purchasers.
Liens on Properties and Purchasers:
When Reuven borrowed money from Shimon, a lien automatically took effect on his real estate, on his lands, similar to a bank that has a mortgage on a property. If Reuven does not pay his debt and claims he has no money, Shimon is permitted to collect his debt from the land. And if Reuven sold his properties in the meantime, the buyers are called "purchasers," and the lender has the ability to go to them and say: You bought this property after I lent money to Reuven, and therefore I have a lien on it - either I will force you to sell it so I can get my money back, or you can pay me yourselves.
The same applies to a bank loan: the bank has a mortgage on the property. Even though the house is in the borrower's possession, he owes the bank the million dollars of the mortgage, and when he comes to sell it to someone else, the bank tells him: I have no objection to the sale, but first you must settle the debt - the first million dollars from the sale money comes back to us, and the house cannot be sold for less than this amount. The point is that the lender has a lien on the property.
The halachah, as we will see in our Mishnah, is that unless specified otherwise - all the borrower's real estate is immediately subject to a lien for the obligation to repay the loan.
Therefore, if the document is returned to the lender on the assumption that it is his - for had it been paid, the borrower would have torn it up - the lender might collect not only from the borrower, but even from the purchasers who bought his properties in the meantime, and he ends up collecting payment from them a second time.
The Concern for Collusion:
Even if the borrower admits and says that he has not yet paid the debt and that the document should return to the lender as an unpaid promissory note, there is a concern for collusion - a cooperative effort of deceit and conspiracy between the borrower and the lender. The two might say to each other behind closed doors: We both know the loan was paid, but the document has resurfaced, and you have no money. Therefore, admit in beis din that the document is still mine, and I will collect the money from the person to whom you sold your house, and we will split the money from the house that I receive. Because of this concern, the Sages established that wherever there are purchasers who might be harmed, we do not return the promissory note even to the lender, lest it be a plot to dispossess those who purchased the properties in the meantime.
The Text of the Mishnah:
The Mishnah deals with a person who found a promissory note flying in the wind. The language is plural - "matza shitrei chov" - even though it refers to a single document, in order to parallel the following mishnayos.
"im yeish bahen achrayus nechasim, lo yachazir" - if it is explicitly written in the document that there are properties mortgaged for this loan, meaning there are purchasers exposed to collection, the finder should not return the document to the lender, lest he collect more than is due to him at the expense of the purchasers.
"shebazeh nifra meihen" - for returning the document will allow the lender to collect money from the purchasers, those who bought properties that were sold by the borrower after the loan was given.
"ein bahen achrayus nechasim, yachazir" - but if there are no mortgaged properties guaranteeing the loan, and there is no concern that purchasers will be exposed to collection, the finder should return the document to the lender, who is presumed to be its owner; for if the loan had been repaid, he would have given it to the borrower, who would have immediately destroyed it.
"she'ein bazeh nifra meihen" - for even when the lender receives the document in his hands, he will not be able to collect money from the purchasers who bought the properties in the meantime, because no lien was written in the document.
"divrei Rabbi Meir" - Rabbi Meir holds that as long as a lien was not written, there is no lien.
"vachachamim omrim, bein kach uvein kach lo yachazir, mipnei shebeis din nifra'in meihen" - the Sages reject his statement: there is no practical difference whether a property lien was written in the document or not, and he should never return the document to the lender. In their opinion, and this is the halachah, when there is no mention of a property lien in the document we say that it is a scribe's error, that the scribe forgot to note the liens, whereas the property lien takes effect regardless.
It turns out that as long as it is not explicitly noted otherwise, the properties are mortgaged, and therefore we do not return the document to the lender, lest the purchasers be harmed. Only in a case where it is explicitly stated in the document that there is no lien on any of the properties - like a loan without a lien - is the document returned to the lender, since no other person is liable to be harmed.
In summary: In this Mishnah, we learned the law regarding one who finds a promissory note. Even though logically the presumption of the document is that it belongs to the lender, we are concerned about a conspiracy between the borrower and the lender that would enable collection from purchasers who bought properties from the borrower after the loan. According to Rabbi Meir, the matter depends on writing a property guarantee in the document, whereas according to the Sages - and as is the halachah - the lien takes effect even when it is not written, and therefore we do not return the document unless it explicitly states that it has no property guarantee.