What are the implications of "Ki Savo'u el ha'Aretz"?
Sifra: It implies that Sh'mitah will only take effect when Yisrael come to Eretz Yisrael proper
Why does the Torah add the phrase "Asher Ani Nosein Lachem"?
Sifra: To preclude Amon and Mo'av. 1
Though it is unclear what the Sifri means, seeing as Yisrael never even entered Amon and Mo'av, and it is something that will occur only in the time of Mashi'ach.
What is the meaning of "Veshavsah ha'Aretz"?
Seeing as "Veshavsah ha'Aretz" implies anywhere in the land
The Yerushalmi leaves this She'eilah unresolved.
What is the significance of the words "Shabbos la'Hashem"?
Ramban: The Torah writes "Shabbos la'Hashem" in connection with Shabbos Bereishis, because Hashem (Kevayachol), rested on it, which is why we may do no Melachah on it. And it uses the same expression regarding the Sh'mitah because it is the seventh year (the Shabbos) of the cycle of years.
Seforno: It implies that, since one is not occupied with tilling the land, one should spend the year in the service of Hashem 4 to a certain degree - even the farmers, who are not on such a high spiritual level. 5
Moshav Zekenim citing the Ramban: Sh'mitah is testimony that the world was created for six thousand years, and the seventh millennia will be Shabbos and eternal rest. Consequently, denying Sh'mitah is akin to denying the Creation and the world to come, like it says about Shabbos. This is why the Torah is so stringent concerning Sh'mitah
Rashi: Where the Torah uses the same expression (in Yisro, Sh'mos, 20:10). See Ramban's objection to this explanation.
See also Oznayim la'Torah DH 'Shabbas la'Hashem".
Moshav Zekenim: Since resting the land improves the quality of the harvest in the ensuing years, as the Torah testifies in Bereishis, 2:2.
Seforno: Just as one is supposed to do on Shabbos, where the Torah uses the same expression (in Yisro, Sh'mos 20:10).
See Seforno on Pasuk 4.
See also Torah Temimah, note 5.