Lirifuas Sarah Chantsha bas Ahava Nechama bitoch shear cholei yisrael.
There is a לא תעשה to work the land in the seventh year. That is a pasuk and uncontested. שדך לא תזרע - The person may not work his field. There is also an עשה that the land should rest [ושבתה הארץ], so if one works the land he transgresses an עשה. What about if he has a goy work his field, is he עובר on the עשה? Is the עשה on the land, that the land should rest, or on the gavra, that HE should rest and not work the land?
That, as you expected, is a machlokes ha-poskim.The minchas chinuch in numerous places [112, 326, 329] says that the mitzva of not working the land in the seventh year is like the mitzva of שביתת בהמתו - having one's animal rest, and devolves on the land. Based on this assumption he was mechadesh two halachos 1] Even if a goy works the land on your behalf it is assur. 2] If one plants at the end of the sixth year and the seed takes root at the begining of the seventh he is עובר. In both instances the land is not resting.
The Chazon Ish was not excited about this idea. He said that the עשה and the לא תעשה are both on the gavra and if a goy works the land or if it takes root after shmitta there is no problem.
The langauge of the Rambam in different places seems to take both positions so there is no definitive proof as to what his shittah was.
The Tosfos Rid in Avoda Zara [15b] first says that the mitzva devolves upon the gavra [שלא יעשה הוא מלאכה בשדהו] and then without blinking the tail lights on his car or making any indication that he is changing direction, says that the עשה relates to the cheftza of the land [דשביתת קרקע הוזהרנו בשביעית ואסור להשכיר לגוי בשביעית] and that one may therefore not rent it out to a goy.
Rav Asher Weiss Shlita [Minchas Asher Simman Aleph] suggests that what the Tosfos Rid means is that the mitzva is indeed on the gavra, however it is not on the מעשה of the gavra but on the תוצאה - the outcome, and thus a goy may not work the field. He gave seven examples of such a phenomenon where the mitzva is not on the מעשה but on the תוצאה and applies to a goy [may I note a delicious אריכות in the topic in the sefer המדות לחקר ההלכה of R' Amiel]. F'rinstence, if a goy immerses your kli, you may make a bracha because your mitzva is not the מעשה טבילה but the outcome, that the kli is immersed that matters. You can have a goy erect a מעקה [fence] on your roof and you make a bracha. A woman who already accepted shabbos on herself and hasn't lit the candles, may have a goy do it and make the bracha herself. A goy can carry out capital punishment on behalf of the Beis Din. In all of these instances, the mitzva is the תוצאה and not the מעשה. So too, the Tosfos Rid holds that the mitzva that the land should rest, relates to the person, but is defined as him making sure that his land lie fallow - the תוצאה.
This approach presents a leniency with respect to the minchas chinuch and a stringency. The leniency is that according to the MC, if a goy works your field, even if he does it for his own purposes, the owner of the field is guilty, for his field didn't rest. According to the Tosfos Rid, if the work was done by a goy without the ratzon of the Jew, it would not be assur, because the issue is that a Jew does melacha through a goy. If the goy does it for himself, then it is not defined that the melacha is being done through the Jew. The stringency may be that according to the Tosfos Rid, when a goy does melacha on behalf of the Jew, not only is he עובר on an עשה but on a לא תעשה because when the Torah forbids us to work the field, it means even through a goy. According to the MC, the לא תעשה devolves on the person while the עשה relates to the field.
There is soooooooooooo much more. The year will be long and if I am zoche and have time will bli neder share a great deal.
A gut Shabbos-a gut Shabbos! A gantza year where every day is Shabbos:-):-)!!