The 138th mitzvah is that we are commanded in this [Jubilee] Year to return all property which has been purchased to its original owners. It is released from the buyer's possession without payment.
The source of this mitzvah is G‑d's statement,1 "[Since the land is Mine, no land shall be sold permanently. You are foreigners and resident aliens as far as I am concerned,] and therefore, there shall be a time of redemption for all your hereditary lands." It is specifically explained for us that the redemption takes place in this particular year in G‑d's statement,2 "In this Jubilee Year, every man shall return to his hereditary property."
Scripture goes into the various details of this mitzvah and explains how the seller settles with the buyer if, before the Jubilee Year, he wants to repurchase the land he sold. It is also explained that this mitzvah applies only to land which is outside the wall of the city.3 Courtyards and houses built in villages are treated like fields and gardens since they were not built within walls.4 They are the batei hachatzerim referred to in the verse,5 "[Batei hachatzerim that do not have walls around them] shall be considered the same as open land; they shall thus be redeemable, and shall be released by the Jubilee."6
The details of this mitzvah are explained in tractate Arachin.7
It too applies only in Eretz Yisrael and only when the Jubilee Year is in effect.