Leviticus 25:32

Overview - Leviticus 25
The sabbath of the seventh year.
The jubilee in the fiftieth year.
14 Of oppression.
18 A blessing of obedience.
23 The redemption of land.
29 Of houses.
35 Compassion to the poor.
39 The usage of bondmen.
47 The redemption of servants.
Treasury of Scripture Knowledge

Leviticus 25:32  (King James Version)
Notwithstanding the cities of the Levites, and the houses of the cities of their possession, may the Levites redeem at any time.

the cities
As the Levites had no inheritance in Israel, but only cities to dwell in; and consequently the houses in these cities were all they could call their own, therefore they could not be ultimately alienated.
Numbers 35:2-8 ; Joshua 21:1-45