1) THE NATURE OF THE REDEMPTION OF "BATEI AREI CHOMAH"
OPINIONS: The Mishnah teaches that the original owner of a house in a walled city may force the person who bought it from him to sell back the house within one year from the day of the sale. During that time, the buyer of the house is required to return the house to the seller if the seller offers to buy it back.
There are two ways to understand how the forced reclamation of the purchased house takes effect.
(a) The NESIVOS HA'MISHPAT (CM 55:1) explains that the Torah enforces an implicit stipulation, a Tenai, in the sale of a house in a walled city. At the time of the sale, the buyer agrees to the unspoken stipulation that the house is not considered to be actually sold to him until one year passes without the seller refunding the money. At that point, the sale retroactively takes effect. However, if the seller does refund the money during the year, then the buyer's money -- which was in the possession of the seller for several days or months -- is considered to have been merely a loan from the "buyer" to the "seller."
(b) The KETZOS HA'CHOSHEN (CM 55:1) understands this law differently. He explains that the sale of the house takes effect immediately, like any other sale. However, the Torah gives the seller the right to repurchase the house by refunding the money to the buyer within a year. The original sale was unconditionally valid, but the Torah imposes upon the buyer the condition that he must nullify the sale and return the house in the event that the original owner wants to buy back the house. (See the following Insight for the ROGATCHOVER GA'ON's additional way to understand this law.)
The Mishnah says that the buyer's use of the house during the first year is considered to be interest on the "loan" that the seller received from the buyer (i.e. the money that the buyer "paid" for the house). According to both the Nesivos ha'Mishpat and Ketzos ha'Choshen, this means that the money is considered a loan if the seller does buy back his house during the first year after the sale. Either the sale never took place and the money paid to the owner was simply a loan in the first place (Nesivos ha'Mishpat), or the sale did take place but it was annulled retroactively when the seller returned the money to the buyer and took back the house (Ketzos ha'Choshen).
The Ketzos ha'Choshen cites support for his view from the RASHBA in Gitin (75a, DH Temiyah). The Rashba writes that the house is indeed "sold" immediately, and the reason why the seller may redeem it within a year is that it is as if the buyer has agreed to allow the seller to take it back if he returns the money within a year.

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2) BREAKING DOWN THE DOOR
QUESTION: The Mishnah teaches that Hillel enacted that when the buyer of a house in a walled city evades the original owner in order to prevent him from buying back his house near the end of the year after the original sale, the original owner may deposit the money in an office (in Beis Din), and then "he (the original owner) shall break down the door and enter" the house.
The Mishnah does not say that he "may" break down the door, which would have implied that he is permitted to reclaim possession of his house by force if necessary. Rather, the Mishnah implies that part of Hillel's enactment is that the owner is supposed to break down the door. What is the point of breaking down the door of the house?
ANSWER: The ROGATCHOVER GA'ON explains this enigmatic statement of the Mishnah as follows:
The Gemara later discusses the question of whether giving an object to a person against his will constitutes a valid act of "giving." For example, if a man says to his wife, "I consent to divorce you, but only on condition that you give me 200 Shekels," and he subsequently refuses to accept the payment because he wants to invalidate the divorce, may she give the money to him against his will? Is such an act of "giving" considered a valid fulfillment of the condition?
The RASHBA (Gitin 75a) asserts that even if payment by coercion is acceptable, it can be acceptable only as an act of giving, but the other party will not acquire what was given. That is, coercion will help only when it is not necessary for the recipient to take acquisition of what has been given. One cannot be forced to receive an object (i.e. to acquire it through a Kinyan) against his will; the recipient (such as the divorcing husband) will not acquire the money that has been thrust upon him. Nevertheless, it can be said that the woman has "given" him the money and fulfilled the condition, since making something available for another person to take is also defined as "giving." In the case of the divorce, the Rashba explains, the condition was for the woman "to give" 200 Shekels to the man. To fulfill this condition, it suffices for her to make the money available to her husband. Even though the money does not become the property of the husband (that is, he did not "receive" it and it did not become his property), nevertheless by making it available to him the woman has "given" it to him.
Accordingly, after Hillel's enactment to pay the buyer of the house against his will (which the Gemara compares to the case of the divorce on condition), the money is considered as having been "given" by the original owner when he throws it into the appointed office, but the recipient (the one who bought the house) does not own that money as long as he does not consent to receive it.
Once we understand that the act of giving something against the recipient's will qualifies only as an act of giving on the part of the giver but not as an act of receiving on the part of the recipient, we must address another question: When the Torah says that the original owner of the house may redeem his house within the first year after the sale, how exactly does this redemption work?
The redemption of a house in a walled city may be understood in two different ways (see previous Insight):
The first way in which the redemption might work is that the Torah enforces an implicit stipulation in the sale of a house in a walled city. At the time of the sale, the buyer agrees to the unspoken stipulation that the house is not considered to be actually sold to him until one year passes (without the seller refunding the money). At that point, the sale retroactively takes effect. However, if the seller does refund the money during the year, then the buyer's money -- which was in the possession of the seller for several days or months -- is considered to have been merely a loan from the "buyer" to the "seller." This is the approach of the NESIVOS HA'MISHPAT (CM 55:1).
The second way in which the redemption might work is that the original sale takes effect immediately, like any other sale. However, the Torah gives the seller the right to repurchase the house by refunding the money to the buyer within a year. The original sale was unconditionally valid, but the Torah imposes upon the buyer the condition that he must nullify the sale and return the house in the event that the original owner wants to buy back the house. This is the approach of the KETZOS HA'CHOSHEN (CM 55:1).
The Rogatchover Ga'on adds his own understanding to the nature of the reclamation of a sold house in a walled city. He explains, like the Ketzos ha'Choshen, that an unconditional sale takes effect at the time of the original sale of the house. However, when the original owner wants to repurchase his house, the sale is not annulled retroactively (as the Ketzos ha'Choshen explains), but rather the original owner must perform a new act of Kinyan in order to acquire the house.
Accordingly, the original owner must do two things in order to reclaim his house: he must pay back the money that he received from the buyer, and he must make the house his property again through a new act of Kinyan.
Depositing the money in the designated office against the will of the buyer suffices to fulfill the requirement to "give" back the money to the buyer (the first condition that the original owner must fulfill), because giving forcefully is classified as "giving." However, based on the Rashba's premise, the money does not come into the possession of the buyer of the house; he has not "received" it. Therefore, the payment cannot constitute a proper Kinyan Kesef, a Kinyan of "payment of money," to allow the original owner to effect acquisition of the property (the second condition). In order for this Kinyan to work, the money must be received by the seller, and not just made available to him. Only when one party receives the money does the other party receive the land in return.
Hence, in order for the original owner to reclaim his house, he must still perform a legal act of acquisition on the property before the end of year. Until he performs that act of Kinyan, the house does not become his again. The Kinyanim of "payment," "barter," and "transferal of deed" are not applicable in this case, because the other party in the sale refuses to cooperate. The only option left is for the original owner to make a Kinyan of Chazakah on his house -- by making an improvement or change in the state of the property. The Mishnah in Bava Basra (42a) describes "locking a door or breaking down a locked door" as an example of such a Chazakah!
This is what the Mishnah means when it states that the process by which the original owner reclaims his house is not complete by merely depositing the money; he must also "break down the door" to his house. He must break down the door in order to make a Kinyan of Chazakah on the house. Only by making a formal act of acquisition (Kinyan) before the end of the year will the house return to his possession. (TZAFNAS PA'ANEACH, Kuntrus Hashlamah, page 4; see also ISHIM V'SHITOS by RAV SHLOMO YOSEF ZEVIN, in the section describing the insights of the Rogatchover Ga'on, 2:15. The same explanation was suggested by the BRISKER RAV (cited by Rav Shmuel Rozovsky in his Shi'urim on Kesuvos 3a) and the EINEI SHMUEL here.)

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