Bava Metzia 66b: If someone sells the future fruits of a date palm to his friend, Rav Huna said: Until they grow, he can back out, but once they grow he cannot back out. Rav Nachman said: Even after they grow he can back out.

בבא מציעא סו ע”ב: המוכר פירות דקל לחבירו, אמר רב הונא: עד שלא באו לעולם ־ יכול לחזור בו. משבאו לעולם ־ אין יכול לחזור בו, ורב נחמן אמר: אף משבאו לעולם ־ יכול לחזור בו.

A Jew in Tiberias who was known as a tzaddik sold half of his mitzvos to a wealthy man for 22,000 rubles. The buyer paid half the amount, but before paying the other half he went to see a rav, who asked Hashem to send him an answer through a dream as to whether this was a worthwhile purchase. He received the answer that this “tzaddik” was not any better than anyone else. The buyer went back to the “tzaddik” and demanded a refund. But the seller refused, and instead demanded the other half of the sale price. The dispute was presented to the Netziv (Meishiv Davar v. 3, siman 14)

The Netziv commented that this seller showed, by the very fact that he sold his mitzvos, that he was not a tzaddik. He brings several proofs:

1. Selling one’s mitzvos is what Esav did. The Torah’s criticism of Esav is not that he sold the birthright for a bowl of soup. The Rashbam says that Esav sold it for its full value, and the soup was only to close the deal. Still, the Torah says that he despised the birthright, because a right-thinking person realizes that spiritual reward is priceless and should not be sold for any money in the world.
2. We see in the Gemara in Taanis (25a) that Rabbi Chanina ben Dosa received a golden leg from his table in Olam Haba and he returned it to Heaven. In Shemos Rabbah (52) we see that his wife was the one who demanded that he return it. So how much more wrong it is to deliberately sell the honor of Hashem, to take pleasure in Hashem in Olam Haba, in order to receive physical pleasure in this world. It is like one whom the king honored with a medal and he sold it to something else. Such a person has despised the honor of the king and deserves a punishment.  
3. The Torah says, “You have cried in the ears of Hashem, saying, ‘We wish we could eat meat! It was better for us Egypt.’ Hashem will give you meat and you will eat.” (Bamidbar 11:18) The complainers remembered the physical pleasures of Egypt, and paid no attention to the spiritual pleasures in the desert, where they enjoyed the revelation of the Shechinah on a constant basis. Therefore Hashem said, I will give you meat until it comes out of your noses, because you rejected Hashem who is in your midst. The same could be said of this man who despised the pleasure of Olam Haba and sold it for physical pleasure, for money. He is deserving of punishment and does not fit the description of a man on a high spiritual level.

Then the Netziv asks on himself from the story (Sotah 21a) of Shimon the brother of Azariah and Rabbi Yochanan of the Nasi’s house, who sold half their reward for learning Torah to those who supported them. He answers that there it is different because they did it in order to be able to learn Torah. The Mishnah in Avos (4:17) says, “One hour of teshuva and good deeds in this world is better than all the life of Olam Haba.” They loved the Torah so much that they didn’t demand full reward for it in Olam Haba, only to take pleasure in Hashem through the battle of Torah. The Gemara also brings the story of Hillel and his brother Shevna, where Shevna offered Hillel money in return for partnership in the reward for his Torah, but a Heavenly voice proclaimed, “If a man gives all the wealth of his house for love, they would despise him.” [Most understand this to be because Hillel had already learned Torah, so the money would not help him learn more; it was merely an attempt to buy off his reward. But the Netziv disagreed with this – perhaps Hillel would continue learning thanks to Shevna’s money.] The money would have helped Hillel learn with a clear mind and no worries. But Hillel was so engrossed in his learning that his poverty did not bother him in the least, as the Gemara tells about him (Yuma 35b) that he used to earn a small coin and give half to the guard at the Beis Medrash door and sit and learn. Therefore he refused to take the money. However, Rabbi Yochanan and Shimon the brother of Azariah knew that if they would have to worry about making a living, it would disturb their learning, as we find in Eiruvin 65a that Abaye said that even a small interruption, like his mother asking him to bring the cereal to the table, would decrease his ability to learn. Therefore they agreed to give up spiritual pleasure in Olam Haba in order to learn diligently. But to simply exchange spiritual pleasure for physical pleasure – that is like the attitude of the complainers in the desert.

The Netziv then takes the position that the sale was not valid, and offers six arguments:

1. The man sold half of his reward, presumably including Torah. But one cannot sell reward for Torah, because that means sitting in the yeshiva in Olam Haba, and only real scholars can do this. As an analogy, there is difference between selling an honor bestowed by a king, which can be done, although it is insulting to the king to do so, and selling a position conferred by the king on a skilled person. One cannot sell the position because the buyer doesn’t have the requisite skills. So once the part of the sale that covers reward for Torah is invalid, the entire sale is invalid.
2. Reward for mitzvos is a דבר שלא בא לעולם which we hold one cannot sell. And even according to the opinion that one can sell דבר שלא בא לעולם, the sale only takes effect when it comes into existence (Bava Metzia 66b), which in this case would be after the seller’s death, at which time no sale can take place.
3. There is no act of kinyan.
4. The Rosh and the Shiltei Hagiborim (in the 8^th chapter of Bava Kama) disagree on whether one can sell the rights to a mitzvah. For example, the mitzvah of bris milah falls upon the father: can the father sell the right to do the mitzvah to someone else? Their disagreement stems from two different interpretations of the story of Yaakov and Esav. The Shiltei Hagiborim holds that Esav really sold the right to offer korbanos to Yaakov, only that he could go back on the sale since there was no kinyan; therefore Yaakov made him swear not to go back. The Rosh holds that the sale was not effective at all. But both agree that if one has already done a mitzvah, one cannot sell the reward to another person.
5. The Gemara in Kiddushin says that a person who regrets doing a mitzvah loses his reward. If it were possible to sell the reward, we could have the absurd situation where the seller could regret his mitzvah and buyer would lose the reward he paid for.
6. The reason why you cannot sell the reward of a mitzvah, says the Netziv, is because the reward comes as a natural consequence of the mitzvah, just as healing is a consequence of taking medicine (Midrash Rabbah beginning of Re’eh, Midrash Tehillim 132). The reason why teshuva helps is that teshuva is a type of medicine too.

Therefore, the sale of the mitzvos was invalid and the buyer can demand back his 11,000 rubles.

Source: Meishiv Davar Chelek 3, Siman 14

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.