"Speak to the Israelite people...the land shall observe a sabbath of the Eternal." (Lev. 25:2)
The idea behind these three sections that appear here - Shmitah (the Sabbatical Year), Dror Avadim Ba-Yovel (The Freeing of Slaves in the Jubilee Year), and Ribit (Charging Interest) - is that the Holy Blessed One is warning Israel not to place their trust in any of those things that appear to people to be trustworthy.
And these three things: Shmitah, Yovel, and Ribit - are Olam (world), Shanah (year) and Nefesh (soul/self/life/body).
Because the purchase of fields and vineyards is an acquisition that people have in the world that they can trust. So to counter that instinct, the Blessed God commands Shmitah, which means that people place their trust in them, and must see that they belong to God. For in the Shmitah year, people do not recognize any ownership in the the things they own. It becomes clear that "the earth belongs to God."
And to counter the instinct that people have to trust in time, that is, when they trust that that will amass wealth by charging for time - which they do when they take ribit - the Torah says, "do not take from him an advance..." (Lev. 25:36). For the essence of ribit is charging a price for time. As it says in the Talmud (Bava Metzia 63b), "The general rule of ribit is: any profit that comes by waiting is forbidden."
And to counter the instinct that people have to acquire the bodies of slaves - so that they can trust in them, and use them for all their needs - to counter this, the Blessed God commanded that we free all slaves during the Jubilee. This is so that people see that one can not own bodies, and so one must send them free. And even while they are slaves, the Blessed God commands that one not work them excessively - and that, too, is so that they understand that another person's body does not belong to them.
So the Blessed God commands these three mitzvot so that it becomes apparent that the world, and the year, and the body are not things that a person should place trust in. For God is the only thing a person can trust.
And that is why, just after these sections, the Torah concludes with "I am the Eternal, your God, who took you out of the land of Egypt, to give you the land of Canaan." (Lev. 25:38) So that it will not seem to a person that their influence in the world has diminished as a result of having been forbidden these things. That is why it says, "I am the Eternal, your God, who took you out of Egypt" - meaning, those benefits which do not come through holiness are actually narrowness and constriction; "to give you the Land of Canaan" - meaning, the benefits that the Blessed God bestows upon us come in holiness, and so we can expand through them. For the Blessed God transmits benefits in holiness, without any constriction.