"But the field of the pasture land of their cities may not be sold"Leviticus 25:34.

The cities that the Israelites grant the Levites each consist of the actual city, surrounding open areas (1,000 cubits to all directions) and then fields around the open areas (2,000 cubits to all directions).

It is forbidden for the Levites to switch these areas. They may not make the space allotted for a city into an open area, an open area into a city, a field into an open area, or an open area into a field.