Bava Metzia, chapter 5, Mishnah 3, deals with additional rabbinic prohibitions surrounding the issue of interest. We will start by presenting the first case outside the text, and afterward, we will focus on the wording of the Mishnah itself.
The First Case: Selling a Field with Partial Payment:
A field owner wants to sell his field now, and the potential buyer does not have the full amount on hand. The buyer gives a down payment, and it is agreed between them that the field will be transferred right now, provided that the rest of the money arrives at a later date. For example: The owner asks for a thousand dollars for his field, and the buyer says to him, "I do not have a thousand dollars right now, and I need a month to get it, but I want to secure the deal. Here is a hundred dollars now, and give me thirty days and I will complete the remaining nine hundred."
The entire transaction is made to take effect from now: The two explicitly stipulate that the acquisition takes effect at this moment, on the condition that the buyer brings the missing nine hundred. And if he does not bring it within thirty days - the transaction will be canceled, the seller will return the hundred dollar down payment to the prospective buyer, and everything will return to how it was.
Why it is Important to Stipulate that the Acquisition Takes Effect "From Now":
The problem we saw earlier in the chapter regarding buying now for future delivery: This could be a rabbinic problem, since the price might go up, and there is an aspect of rabbinic interest in this.
The problem of asmachta, which we will discuss later.
The Mishnah establishes that this is forbidden. And what is forbidden? Certainly it is permissible to give a down payment now and complete the payment later - this is a normal commercial transaction. The prohibition is eating the produce, the crop of the field, until it becomes clear whether the prospective buyer will pay the nine hundred or not.
And to explain the matter:
If the current owner eats the produce: Once he receives the missing nine hundred, it turns out that the buyer has fulfilled all his conditions and the field becomes his. And what did the original owner receive? The full thousand he asked for, and in addition to that, he enjoyed the produce that grew in the field in return for his waiting for the payment - and this is Torah-level interest.
If the prospective buyer eats the produce: Let us assume he did not manage to get the nine hundred. At the end of thirty days the sale is canceled, and the seller returns his hundred dollar deposit to him. It turns out that he gave out a hundred for thirty days, received it back, and also enjoyed eating the produce while waiting - and this is like interest.
For this reason, eating the produce is forbidden. The solution is that the acquisition takes place, but until the matter becomes clear - until it is decided in thirty days who the owner is - neither of them eats the produce. The produce growing in the field, or its value, is placed in the hands of a third party, and he watches over it and hands it over to the true owner when it becomes known who he is.
Question: But is there not only a concern for rabbinic interest here?
One can ask: At most, we are faced with a concern for rabbinic interest, and who said it will even happen? The parties are considering the deal, and why should we not permit the buyer to eat the produce in the meantime? If he does not manage to get the money in the future - indeed, rabbinic interest will be created; but everyone assumes he will get the money, and in such a case there was no problem here at all, since he ate his own produce, which he paid for at the beginning of the process.
The Answer - "One-Sided Interest":
There is a separate rule that "one-sided interest" is forbidden. When a transaction is uncertain, as it may lead to interest or it may not, depending on what happens later, it remains forbidden as long as one of the possibilities could lead to interest. This is the "one side", meaning one of the two possible outcomes. Therefore, it is forbidden to do something that might lead to interest until it becomes clear how things will develop. This concludes the first part of the Mishnah.
The Second Part of the Mishnah: A Collateral "From Now":
This case is not about a partial payment that will be completed at a later stage. Rather, it discusses a person who gives money and says: "This is an interest-free loan, but if you do not pay by the due date, you are transferring a sort of collateral to me retroactively from right now." The Mishnah says: "Im i atah nosen li mikan v'ad shalosh shanim, harei hi sheli" - "If you do not pay me between now and three years, it is mine." This means that if you do not return the million shekels to me within three years, the entire farm is mine, "me'achshav" - from right now. The borrower stated this explicitly: the transaction takes effect retroactively from that initial moment if he defaults.
This stipulation of "from now" is required for several reasons. The first is what we already learned in the previous chapter: one may not pay money now for a future supply of goods, due to the law of "poseik al hapeiros" - setting a price on produce, which functions like a futures contract and is rabbinically forbidden. Here, however, this is not the case. The acquisition takes effect right now if the condition is met that the money is not repaid. If the money is indeed repaid, the condition is not met, and there is no transfer of ownership of the field.
Therefore, the Mishnah states "harei hi shelo" - it is his. The acquisition is valid, and the estate is transferred to the lender's possession and ownership if the borrower does not repay his debt within the designated three years. This is not an interest-bearing loan, because it practically functions as a retroactive sale: I am lending you money, and if you do not return it, it is no longer a loan but a sale retroactively from this moment, and I am purchasing your field completely. Even if the loan is for one million shekels and the field is worth ten million, it does not matter, since the laws of overcharging do not apply to real estate. The transaction is structured as a retroactive sale in the event the debt is not paid, and therefore it is completely permissible.
Asmachta:
An asmachta is a situation where a person says something merely to close or advance a deal, but does not truly mean what he says. If the borrower had said: "Lend me the million shekels. I am a wealthy man and will repay you in three years, and if not, you can take my entire farm," we would say that this is an asmachta. By saying "you can take my entire farm," he does not actually mean it, because he fully intends to repay the debt and does not believe it will ever come to that.
However, since the borrower said "me'achshav" at the time of the transaction - "if I do not pay you within three years, my entire estate is yours from this moment" - this is not mere rhetoric or words casually thrown in to close a deal. Rather, his intention is that halachically, the estate will actually transfer to the lender's possession right now if he fails to pay. Therefore, there is no concern of asmachta here, and the acquisition is valid and binding.
The Mishnah concludes: "V'kach hayah Baisos ben Zunan oseh al pi Chachamim" - and this is what Baisos ben Zunan would do with the approval of the Sages. Baisos ben Zunan was a sort of pawnbroker, and this is exactly how he operated. In the presence of the Sages and with their approval, he would give loans and take collateral, saying: "If you repay me, you will get your collateral back, and if not, it will be mine." And this was how he made his living.