Mishnayos Eruchin Perek 9 Mishnah 3

One who sells a house from among the houses of walled cities may redeem the house immediately, even without the consent of the buyer, and he may redeem the house during the entire twelve months following the sale, but not after that. When he redeems the house within the twelve-month period, he returns the sale price to the buyer, and this is ostensibly like a form of interest, as the buyer has effectively resided in the house for free in exchange for the fact that the buyer’s money was in the possession of the seller. It is not considered interest, because the buyer owned the house during the period in which he resided in it. If the seller died, his son may redeem the house from the buyer. If the buyer died, the seller may redeem it from the possession of the buyer’s son. If the buyer sold the house to another, one calculates the year only from the time that the owner sold the house to the first buyer, as it is stated: “And if it is not redeemed until the passage of a full year for him, then the house that is in the walled city shall stand in possession of the one who bought it in perpetuity” (Leviticus 25:30). The term “for him” indicates that the year is calculated from when the initial owner sold the house. When it says: “A full year,” this serves to include the intercalated month in the year calculated from the sale, if it was a leap year. Rabbi Yehuda HaNasi says: The word “full” serves to give the seller a year and its addition, i.e., the year during which the house may be redeemed is not the 354-day lunar year, but the 365-day solar year.