Erchin 9:3-4

Erchin 9:3

If a person sells one of the houses in a walled city, he can redeem it immediately and for up to 12 months. This kind of like interest (since he must pay the entire purchase price to redeem it, without prorating) but it is not interest (because it’s not a loan). If the one who sold it died, his son can redeem it; if the one who bought it died, it can be redeemed from his son. The year is only counted from the original sale because Leviticus 25:30 tells us “until one full year for him,” “full” coming to include the extra month inserted in leap years. Rebbi says that one is always given a regular year of 12 (lunar) months plus the extra days of a solar year.

Erchin 9:4

If the end of the 12 months arrives and the seller hasn’t redeemed the house, the buyer takes permanent possession of it. The same rules apply to a sale and a gift because it says “in perpetuity.” Previously, a buyer would hide on the last day of the 12th month so that he could take permanent possession of it so Hillel the Elder enacted that the seller could deposit the redemption money with the Temple treasurer then re-enter the house by breaking down the door. The buyer could then go and claim the redemption money whenever he wished.