HETER MECHIRA (the Sale License) refers to produce from Jewish fields that were formally sold to a non-Jew before SHEMITAH, the way Chometz is sold to a non-Jew before Pesach. The sale of the land in Israel of participating farms allows work to be conducted in the fields. The fruit and vegetables, grown on these fields, are considered as grown on non-Jewish land and therefore are not bound by the restrictions of SHEMITAH.

It is a Mitzvah to be Melamed Zechut (speak in defense of) the Jewish People in the Land of Israel. According to the Rambam, SHEMITAH today is only Rabbinic and not mandated by Torah Law. Only when the majority of the Jewish People will reside in Israel will SHEMITAH once again become a Torah Mitzvah. We're getting close to that point, but we're not there yet.

Thus, HETER MECHIRAH is not something forbidden or barely permissible. Rather, it is clearly permissible with a strong basis in Halacha, just like the sale of Chometz before Pesach. The greatest Sages supported HETER MECHIRAH including Rabbi Yitzchak Elchanan, Rabbi Yehoshua of Kutno, Rabbi Yehoshua Leib Diskin, Rabbi Naftoli Herz, the Chief Rabbi of Yafo, the Sochatchover Rebbe, Rabbi Shmuel Moholiver, The Rishon L'Tzion Rabbi Yaakov Shaul Elyashar, Rabbi Yosef Engel, HaRav Avraham Yitzchak Kook and Rabbi Ovadiah Yosef.

Therefore Rabbi Tzvi Yehudah Kook wrote in Lenetivot Yisrael that if someone does not wish to use the HETER MECHIRA, he is just choosing a higher level of strictness and a degree of extra piety.
We have to be very careful about a measure of extra piety lest it lead to sin, as Ramchal explains in his Mesilat Yesharim, Ch. 20, On Piety.

We have to examine well if our not availing ourselves of the HETER MECHIRA is not a stringency that will lead to a leniency. Indeed, numerous leniencies may be involved in the refusal to make use of the HETER MECHIRA – for example:
1. Harming the ability of Jews to earn a living in the Holy Land as it says, "Help your brother survive" (Vayikra 25:36). We have to be concerned that our fellow Jews do not become impoverished.
2. "Buy from your friend." (Vayikra 25:14) If you have the possibility of buying from a Jew or a non-Jew, you have to prefer a Jew, for he is our brother.
3. "Libelling the decisions of our predecessors" (Ketubot 103). If someone uses the expression that it is forbidden regarding produce, provided in accordance with HETER MECHIRA, he is libeling the great Rabbis who followed it. (See Lenitvot Yisrael 2:224)
4. Not scorning others: A major precondition for achieving Piety is not scorning others who do not behave as we do (Yerushalmi Berachot: 9). The use of the expression, "forbidden", scorns those Jews who eat the produce of HETER MECHIRA (See Lenetivot Yisrael).
5. Not undermining Rabbinic authority: The Talmud tells us that Rabbi Tarfon took the strict approach and recited the Shema like Beit Shamai. The Rabbis said to him, "You deserve punishment, for you violated the words of Beit Hillel."(Mishnah Berachot 1:2) This is puzzling considering that Beit Shamai is stricter. So how did Rabbi Tarfon violate the words of Beit Hillel?

Ramchal therefore explains, "The controversy between Beit Shamai and Beit Hillel weighed heavily on Israel. It was finally decided that the Halacha follows Beit Hillel. This constitutes upholding of the Torah. G-d forbid, that the Torah should become two Torahs. Therefore, according to this Mishnah, it is correct to hold like Beit Hillel, even if they are more lenient, rather than to be strict like Beit Shamai." (Mesilat Yesharim, Mishkal HaHasidut, Ch. 20.)

Thus, we may rely on the HETER MECHIRA, which will strengthen the economy and agriculture of the Jewish People in the Holy Land.