Parashas Behar · Aliyah 5 · Passuk 25:29
Chabad.org — English translation + Rashi
Torah Temimah cites Sifra / Arakhin 31a.
English: “A dwelling-house in a walled city.” This teaches that this law applies only to houses of walled cities.
Classical derasha (Sifra / Bavli Arakhin).
The special one-year redemption window applies only to houses inside cities that were walled from the time of Yehoshua.