פירוש על קידושין 41:13

The Talmud responds that the similarity between houses in a walled city and open fields is that relatives have an obligation to redeem both of them (as long as they have the ability to do so). This accords with R. Eliezer, who holds that whenever relatives can redeem land or houses sold by their family, they must do so. R. Joshua holds that it is an option, not an obligation.
This allows us to restore R. Sheshet’s answer—relatives may not redeem houses in walled cities.