Gemara

15b: One is commanded that his field rest in Shemitah;

(Mishnah - Beis Shamai): One may not sell a plowed field in Shemitah to someone suspected of working in Shemitah;

Beis Hillel permit this. He may assume that the buyer wants to leave it fallow until the following year.

62a (Mishnah): If Reuven told a worker (Shimon) 'here is a Dinar. Gather Shemitah Peros for me for it (the Dinar)', the money is forbidden;

If he told him 'here is a Dinar. Gather Shemitah Peros for me', it is permitted.

(R. Avahu): Chachamim fined to forbid wages of donkey-drivers.

(Beraisa): If donkey-drivers were working with Shemitah Peros, their wages are Shemitah.

Question: What does it mean 'their wages are Shemitah'?

Answer #1: The employer pays them with Shemitah Peros.

Rejection: One may not pay a debt with Shemitah Peros. They are "Le'achlah", and not for business!

Answer #2: Rather, their wages have Kedushas Shemitah.

Rejection (Mishnah): If Reuven told a worker 'here is a Dinar. Gather Shemitah Peros for me for it', the money is forbidden. If he told him 'here is a Dinar. Gather Shemitah Peros for me', it is permitted.

Defense of Answer #1 (Abaye): Really, he pays them with Shemitah Peros, in the permitted way (for a gift).

(Mishnah): Levi may not tell Yehudah 'take this Ma'aser Sheni to Yerushalayim, we will divide it there' (this is like paying for his labor with Ma'aser Sheni). He may say 'take it to Yerushalayim. We will eat it there', and they give to each other a free gift.

Defense of Answer #2 (Rava): Really, their wages have Kedushas Shemitah. A regular worker's wages (for picking Shemitah Peros) are permitted. Chachamim decreed only about wages of donkey-drivers, which are great.

Rishonim

Rambam (Hilchos Shemitah 6:12): If one told a worker (Shimon) 'here is a Dinar. Gather vegetables for me today', it is permitted. It is not like Shemitah money. Shimon may use the money for whatever he wants. If he told him 'gather vegetables for me for it (the Dinar)', it is like Shemitah money. He may buy with it only food and drink, like (money of) Peros Shemitah.

Rashi (62 DH Lekot): 'Gather for me for it' a sale, i.e. gather a Dinar's worth of fruit for me. One may not keep the money after the time for Bi'ur. 'Gather for me' connotes that he hires him, and not a sale, so there is no value to receive Kedushas Shemitah.

Ri Korkus: The Yerushalmi calls the first law 'a Halachah of Imum.' There was no clear reason to permit, but they demurred and permitted.

Radvaz: In the Mishnah, 'permitted' means that it has no Kedushas Shemitah. 'Asur' means that one must eat it with Kedushas Shemitah. The Yerushalmi asked what is the difference between the two expressions, and said that it is a leniency of Shemitah. The Rambam connotes that it should be permitted (in both cases), but Chachamim made a fine only in the Seifa. What is the reason? Perhaps in the Reisha he is hired for the entire day for an Isar, so he receives wages for the day, and there is no reason to fine him. In the Seifa he is not hired for the entire day. Perhaps in a half hour he will gather vegetables worth an Isar. He is not hired for the day, rather, for the vegetables, so (it looks like a sale, and) there is reason to fine him.

Kesef Mishneh: The Rambam holds that we discuss vegetables not normally planted. Regarding vegetables normally planted, even if they grew in the sixth year, they are all forbidden in Shemitah (Rambam 4:3). The Mishnah supports this. It says 'if one bought field vegetables, which connotes that they grew by themselves.

Rambam (13): If donkey-drivers do forbidden work in Shemitah, e.g. they brought too much, their wages are forbidden. This is a fine. Chachamim fined donkey-drivers, but not (regular) workers, for workers receive small wages just enough to live on.

Radvaz: In the Yerushalmi, R. Heila says that 'the wages are Shemitah' regarding donkey-drivers and Ketefim (people who carry on their shoulders), just like they fined people who work with Yayin Nesech. How does the Rambam know that Ketefim receive more than needed to live on? Perhaps Stam workers long (alternatively, risk their lives) for their wages, but Ketefim do not.

Ran (30a DH Garsinan (2)): Granted, regarding Yayin Nesech Chachamim fined workers who transgressed, for they want the Yayin Nesech to exist (to profit through it). Why did Chachamim fine regarding Shemitah? Perhaps also here, it is when the worker transgressed. The Yerushalmi says so. Also the Rambam says so. He holds that the Gemara concludes that Chachamim fined donkey-drivers and regarding Yayin Nesech, i.e. because these Isurim are Tofes Damim (the Isur transfers to what it is exchanged for). The Gemara connotes that the wages are forbidden to everyone. If not for Tefisas Damim, there would be no reason to forbid others who did not transgress. Therefore, we cannot learn to cases of people who worked with other Isurim, e.g. one who dyed with Orlah or Kilai ha'Kerem. Perhaps the fine is only when we decree wages due to Chalipin (what is exchanged for the Isur). One could distinguish and say that regarding other Isurim, we fine to forbid only to the workers. For Shemitah and Yayin Nesech, we fine the workers and others alike, lest people say the same about money paid for them. However, since this Isur is mid'Rabanan, we do not seek stringencies that Chachamim did not teach about.

Perush ha'Mishnayos (Shevi'is 8:4): One may exchange Shemitah Peros, e.g. 'take this Pri, and I wil take Pri Ploni.' One may buy a loaf from a baker, and say 'I will bring to you field vegetables when I pick them.' However, if he bought a loaf for money, and did not specify that he will bring field vegetables,it is a debt, and one may not pay a debt with Shemitah Peros.

R. Shimshon (Perush ha'Mishnah): The Heter to buy a loaf in exchange for field vegetables that he will pick, is because it is like a free gift, for field vegetables are available. The Yerushalmi says that R. Yehudah and R. Nechemyah forbid, because (they hold that) field vegetables are not available.

Rosh (Perush ha'Mishnah): The Yerushalmi says that this is a Halachah of Im'um, i.e. it was proper to forbid them, but they demurred and permitted. Reish Lakish answered that the employer shows the worker where the vegetables are. He himself could take them; the worker merely brings them for him. A normal case would be like business, and the money would be forbidden. R. Yochanan permits even if he did not show him. This is a leniency of Shemitah.

Poskim

Shevet ha'Levi (2:198 DH v'Af): The Minchas Chinuch (112) learns from Avodah Zarah 15b that one who rents land to a Nochri who will work it in Shemitah transgresses the Aseh to let the land rest. Even he would agree that the Isur transfers to the money only when one sell Peros. When one tells a worker 'gather Shemitah Peros for me today', the wages are permitted. The same applies to all Isurei Hana'ah; the money is permitted. We should not be more stringent about renting land. We should be more lenient, for the money is not for the Aveirah. It only causes the Aveirah. It is difficult to say that we forbid the rental because it is great, like we forbid wages of donkey-drivers. We do not find that Chachamim decreed about this. However, if one rented his land to Nochrim, he must forfeit a fifth of his money in order to fulfill an Aseh, i.e. to redeem his land. Pischei Teshuvah (YD 157), holds that a passive Aseh is like a Lav, so one must spend all his money (if needed) to buy it back! Perhaps we can be lenient nowadays that Shemitah is mid'Rabanan, and some Poskim say that rental acquires only to be stringent. Also, perhaps we do not hold like the Minchas Chinuch. It seems that Tosfos (15b DH Mi) and the Maharit (YD 2:52) disagree. One must return the rental to get the field back, but perhaps he need not return more than this.