Bava Metzia 10b: There is no agency for sin.
בבא מציעא י ע”ב: אין שליח לדבר עבירה
During the Shmitah of 5761, one religious kibbutz decided to invent a heter mechirah that would work even according to the Chazon Ish. The Chazon Ish objected to the regular heter mechirah done by the chief rabbinate for four reasons: 1) Since it is forbidden to sell land to a non-Jew in Eretz Yisroel, the representative carrying out the sale on behalf of all the land owners is a שליח לדבר עבירה – a representative to commit a sin – and his representation is invalid. 2) The sale is not serious since it is not accompanied by a professional survey and appraisal. 3) The sale is not legally binding since it is not registered with the government. 4) They make a condition that the non-Jew must sell it back after Shmitah, which renders it a temporary sale, and a temporary sale is like a קנין פירות which does not remove the restrictions of Shmitah.
(It would seem that these objections are mutually exclusive and asked as a ממה נפשך type reasoning: if anyone finds a way to disagree with arguments 2, 3 and 4 and believes that the sale is 100% valid, then it is a sin and one cannot appoint a representative to commit a sin.)
This hechsher had the land owners carry out the sale themselves, they used professional appraisals, they registered it with the government and they made no conditions. They drew up a separate contract to sell the land back, which would go into effect after a year. A reliable hechsher endorsed their products.
Some customers in America, surprised to see Shmittah produce on their supermarket shelves, approached Rabbi Yisroel Belsky and asked him whether to buy it.
Upon investigation, Rav Belsky discovered that the land was owned as a general partnership by all members of the kibbutz, but only the officers of the community directly participated in the sale. One partner acting on behalf of the others is still shlichus (agency) and so the first objection of the Chazon Ish was never really eliminated.
Furthermore, many poskim object to the heter mechirah on different grounds than the Chazon Ish. They argue that the sale is not real since the Jewish owner continues to control all decisions regarding the land’s usage, sells the produce and keeps the profits earned. This is similar to one who sells his property on paper to prevent his creditors from seizing it, but continues to act as the owner (Choshen Mishpat 99:7). (Contrast this with the sale of chometz, where the Jew has nothing to do with the chometz until after Pesach.) The heter mechirah done by this kibbutz was subject to this same objection.
And of course, the sale of land only solved the problem according to the practice of Yerushalayim that fruit grown on non-Jewish owned land does not have holiness of Shmittah. People in Bnei Brak, on the other hand, are stringent about this.