The shemittah year is over, but I figured I'll publish some notes on Heter Mechira. Its based on an article in Journal of Halacha and Contemporary Society and on the book Shemittah and Yovel by Dayan I. Grunfeld. Its mostly accurate.
In the early 1900's, the frum residents of Israel would have faced serious hunger problems if they couldn't work the land on shemitah. They came up with a heter mechira in which they would sell the land to a gentile, and have gentiles do some work for them. Nowadays, the farms would lose much international business if they didn't produce food every 7th year. Therefore they use the heter mechira and work the land themselves. For the heter mechira to be muttar to do, all of the following halachic leniencies must be true. (It may be easier for the shemitah food to be kosher to eat.)
1. Shemitah nowadays is 100% d'rabanan. Other parts of the heter rely on this.
Although there aren't that many views that dispute this, it may be that shemitah should be considered like a d'oraysa because it was accepted with an “curse & a oath ” [see Nechemia 10].
2. Selling the land uproots the kedusha from both the land and the fruit. (Otherwise there would be no point.)
Although even many haredim in Israel are lenient, its not such a simple issue. The main machlokes is between R' Yosef Karo and the Mabit. There is a teshuva of the Rambam that supports the Mabit.
Correction: Many people are lenient and treat peiros nachri without kedushas shevi'is. But almost everyone holds that it is forbidden for a Jew to work on land owned by a gentile. The hetter mechira relies on a minority opinon that permits it.
3. A “Ha'arama Nikeret” is muttar.
This isn't such a serious problem, especially since shemitah is d'rabanan.
4. It is permitted to temporarily sell Jewish land to a gentile.
This is where the heter runs into serious issues. If the sale is real, it should be forbidden. Even renting land to a gentile is rabbinically forbidden! The HM proponents have 2 responses. 1.A temporary sale is permitted yet still works to uproot the kedusha. 2. Its permitted to sell land to Muslims.
Its questionable who exactly the issur to sell land to applies, but an accurate girsa of Rambam Perush HaMishanyos includes all gentiles.
5. The sale is considered a real sale.
If you sell a whole country to one Arab, but there's no way he'll be able to use it in any way, is the sale real? The government doesn't recognize the sale, and Dina d'Malchusa Dina should make it void.
Some Heter proponents rely on the opinion that Dina d'Malchusa Dina doens't apply in Eretz Yisroel because every Jew has a portion in the land.
Other issues
Shemitah was accepted during much harder times and kept through much worse conditions. They never tried such a heter then. A midrash talks of the gentiles who mock the Jews for eating thorns during the shemitah year.
As Israel got richer, the one-time heter became even more lenient. What was once a horas sha'ah to allow non-Jews to do issurei d'rabanan grew into the heter today where shemitah is treated like any other year.
Other Kulas
R' yannai told the people to work the land during shemitah to pay their tax. Perhaps it was similar hardships as now.
Tosafos suggests it was actual pikuach nefesh. The historic evidence shows that a cruel Roman general was waging war against the Persians, and he probably wouldn't have exempted the Jews from their taxes!
According to the Ba'al HaMa'or, shemitah is voluntary!
I would say this das yochid is cancelled out by the da'as yochid that shemitah is d'oraysa. If your choshesh for one, you should be choshesh for the other.
There is some question what the exact year of shemitah is. Perhaps that safek can help to be lenient.
Safek d'rabanan l'kula doesn't apply if it would be mevatel the mitzvah.
The Ironies.
On many issues, each side takes the opposite views of what they usually would hold so they can be for/against the heter mechira. (Or they keep to their veiws and refrain from using an argument.)
1. You can sell sell land in Israel to Muslims.
2. Dina d'Malchusa Dina doesn't apply in Eretz Yisrael.
You wouldn't here the DL community make these argument in other contexts, and many don't even use them for heter mechira.
3. Manuscripts show the girsa in the Rambam was really goy, not akkum.
4. The historic evidence shows Tosafos was right when they said it was pikuach nefesh.
Charedim don't like basing halachos on more accurate manuscripts, and definitely not on historic evidence.