פירוש על עבודה זרה 41:3

The Mishnah assumed that it was more obvious that one cannot sell fields to a non-Jew than it is that one cannot sell them a house. So why is it more obvious? There are two reasons not to sell them a field—1) It gives them a foothold in the land; 2) The produce becomes exempt from tithes. But there are also two reasons not to sell them a house—1) It gives them a foothold in the land; 2) The house becomes exempt from a mezuzah. So why is it more obvious with regard to a field than a house. R. Mesharsheya answers that there is no obligation for houses in Israel to have mezuzot. The obligation falls on the person, not on the house.