Arachin 31 - Redemption of Houses
If one sells a house in a walled city (surrounded by a wall in the time of Joshua), he can redeem it immediately, without any waiting period, or any time afterwards, for a full year. It thus looks like prohibited interest: the seller got the money, the buyer lived in the house (interest), and then the seller returned the money (as if returned the loan). However, it is not really prohibited interest, since interest is a loan payment fixed in advance, and here he sold his house, and only later it turned out that he could buy it back.
What does the Torah mean by saying that the redemption period is a "full" year? - If this year happens to have thirteen months, he is given an extra month. (To adjust for difference between lunar months and solar year, an extra month is added every few years). Rabbi Yehudah the Prince says that "full" year means a solar year of 365 days. It can thus happen, according to the Sages, that a house sold, for example, on the fifteens of the first month of Adar I, will have more redemption time than another house, sold later, on the first of Adar II.
Art: Paul Cezanne - The Manor House At Jas De Bouffan