Gemara

(R. Yochanan ben Zakai): The Maneh (and Shekel) of Hekdesh is (are) double a standard Maneh.

Sotah 21a (Ula) "Boz Yavuzu Lo (they will scorn him)" does not refer to Shimon, the brother of Azaryah. (He was called so, for his brother supported him.) nor to R. Yochanan (who was supported by the Nasi). Rather, it refers to Hillel and Shevna.

(Rav Dimi): Hillel engaged in Torah. His brother Shevna engaged in business;

Shevna: Let us be partners, and split the rewards (in this world and the next).

A voice from Heaven: "If a man would give all his wealth for love (Torah learning), they will scorn him."

Pesachim 53b (R. Yochanan): People who are Matil Mal'ei l'Kis Chachamim (help them profit in business) merit to sit in the Heavenly Yeshiva - "Ki b'Tzel ha'Chachmah b'Tzel ha'Kasef."

Rishonim

Rashi (Zevachim 2a DH Shimon): Azaryah stipulated with Shimon that he will supply Shimon's needs, and receive a share in the reward for his Torah.

Maharsha (21a DH Mai): Shimon and R. Yochanan were scorned in this world (they were called by the name of their supporters) and the next (they lost half the reward for their learning. Ula teaches that one should not do like Shimon or R. Yochanan. Rather, he should refuse, like Hillel.)

R. Yerucham (Toldos Adam v'Chavah 2:5, cited in Bedek ha'Bayis YD 246): David can stipulate before learning that Levi will support him and get half the reward, like Yisachar and Zevulun. David cannot stipulate after learning - "if a man would give all his wealth...", like it says about Hillel and Shevna. The Meforshim say that David loses (if he stipulates); he forfeits his share.

Poskim

Shulchan Aruch (YD 246:1): One who cannot learn, because he does not know how to learn at all, or due to distractions, should support others who learn.

Rema: It is considered as if he himself learned. Levi can stipulate with David that David will learn and Levi will supply his income and share the reward.

Shach (2): They share the reward, and also the money that Levi earns.

Gra (7): Bereishis Rabah (99:11) says that Zevulun is mentioned before Yisachar in Yakov's Berachah because he did business and fed Yisachar, who learned. Moshe said "rejoice Zevulun in your going out (because you have a share of) Yisachar in your tents." Shimon is called 'Azaryah's brother', even though he was greater than Azaryah, because Azaryah supported him.

Maharam Alashkar (101, cited by R. Akiva Eiger): Rav Hai Gaon said that it is folly to think that one can sell the reward for fasts or Mitzvos that he did. "Tzidkas ha'Tzadik Alav Tihyeh v'Rish'as ha'Rasha Alav Tihyeh" - just like one is not liable for another's sins, one does not receive another's merits. Is reward for Mitzvos something a person can carry around and give to whom he wants?! Reward is honor for one's deeds. One receives the Shechinah; he is praised and stands in the place appropriate for him. If one sold the reward for his fast, he lost it. He did not fast for Hash-m, rather, for money. Perhaps he will be punished for making Hash-m's name a mockery and an axe to use for food. However, surely one who pays teachers to teach, and helps support those who learn so they will be free to learn, has a great reward. One who tries to buy another's reward is disgraced, like Shevna.

Esh Da'as (Parshas Vayelach, cited by R. Akiva Eiger): One who paid 100 gold coins for another's share in Livyasan should not rejoice. The seller should mourn having made Olam ha'Ba a joke, and sold eternal life for money. Eating from Livyasan is not physical. It is an eternal delight due to one's Mitzvos. Once he sold his share, he lost it. The buyer gets reward for showing how dear is Olam ha'Ba to him, that he spent a vast sum for it. "B'Tzel ha'Chachmah..." refers to one who gives half his income to the Chacham. He gets half the reward, like Yisachar and Zevulun - "Shneihem Mele'im Soles" (Bamidbar Rabah 13:17). "Ashrei Enosh Ya'aseh Zos" alludes to supplying a Chacham's needs, like Azaryah did for Shimon. "La'Machazikim Bah" refers to one who gives after the Chacham learned. He does not get the reward of Torah, just the Mitzvah of supporting Torah.

Ohr ha'Chayim (Shmos 30:13 DH v'Omar): The verse says "he will give a half-Shekel Kodesh" to teach about one who is not a Ben Torah. He gives Kofer Nafsho, like Yisachar and Zevulun, and Shimon and Azaryah, and many like them who give half their money to those who toil in Torah. Perhaps b'Shekel ha'Kodesh alludes to Chazal's teaching that the Shekel ha'Kodesh is double. The one who learns will not get only half his reward. The Shekel ha'Kodesh is double. Each of them gets a full Shekel (the full reward for the learning).

Aleinu Leshabe'ach (Shmos p.611): R. Shmu'el of Salant said similarly, and that one can explicitly stipulate in the document that Yisachar does not lose from his reward.

Halichos v'Hanhagos (p.71): The Chazon Ish said that Yisachar does not lose from his share.

Minchas Yitzchak (7:87:3): If Reuven stipulated to support Shimon, and then Levi supports Shimon without a Tanai, what is the law? A Chacham's wife shares the reward of her husband's learning. Will we say that he sold his half, and receives nothing?! The Chida (Rosh David, Parshas Kedoshim) says that what Shimon's wife receives does not detract from his share. He does (learns) with intent for himself, and he did not stipulate about this. Hash-m, in His Chesed, gives half of her husband from His (storehouse of reward). This is unlike a Chacham who stipulated. If so, also if Levi supports without a stipulation, this does not decrease Shimon's share. Therefore, Reuven cannot protest. This is if Reuven supplies all Shimon's income, and gets half of Shimon's learning. If Reuven supplies only part of Shimon's income, and gets half of the percentage of Shimon's income that he supports (Chayim Sha'ul 2:38:44), then Shimon can make a partnership with others to receive half the reward of what remains for supplying the rest of his income.

Minchas Yitzchak (DH v'Gam): The Ohr ha'Chayim and Rosh David (Mishpatim, unlike he said in Kedoshim) say that the Chacham does not lose anything from his share. Surely they hold that one cannot make a second partnership. It is a Chidush that Hash-m does this Chesed. We cannot expect Hash-m to do it a second time. Tuv Ta'am v'Da'as (2:217) says that Olam ha'Ba (the world to come).is not wages, like Chazal say (no one did a Mitzvah until Hash-m gave to him the means to do it). Rather, it is Chesed. The Ramban (Devarim 6:26) says that it is called Tzedakah, for a master need not pay his slave. Hash-m rewards the supporter, for the learning is due to him. If Reuven supplies all of Shimon's needs for a small part of the reward, Shimon cannot share his reward with others, for they do not enable him to learn. If Reuven supplies part of Shimon's needs for half the reward, perhaps Shimon can share part of his half with others, for they enable him to learn.

Hafla'ah (Kesuvos, introduction, 43): Some who generously support Torah think that they buy part of his Torah, like other acquisitions. Chalilah to think so! Would a Chacham sell even one hour of Olam ha'Ba for all life in this world, and all the more so an hour of Torah, which is worth more than all life in Olam ha'Ba. Chazal said "Orach Chayim Pen Tefales" (Mishlei 5:6) - one should not do business with Torah and Mitzvos. "V'Rachok mi'Peninim Michrah" (the first verse of Eishes Chayil) - one cannot buy it even in Olam ha'Ba. Everyone receives his full reward. This is like lighting (a Ner) from a burning Ner. The original Ner does not lose from its light. Likewise, those who support Torah receive reward from Hash-m, and do not detract Chas v'Shalom from the Chacham's portion. One who says 'mine is yours and yours is mine' (Avos 5:10) is an ignoramus. I.e. my money is yours, and your Torah is mine. The Mishnah in Pesachim (89b) alludes to this. If David appointed Levi on his share of a Pesach, David's partners (in the Korban) may give him his share. He eats by himself, and they eat by themselves. I.e. if one cannot engage in his portion in Torah (his share in Olam ha'Ba), and he appoints others to learn for him and he finances him, they did not sell their Torah for income in this world, Chas v'Shalom. They can give to him his share and he eats his share in Olam ha'Ba, and they eat theirs, without any loss of reward. It says "Semach Zevulun b'Tzeisecha (v'Yisachar b'Ohalecha)", for each delights in his. Neither takes the share of the other.

Tzitz Eliezer (20:34): In Rishon l'Tziyon (YD 246:11), the Ohr ha'Chayim says that if two stipulate that one will learn and the other will work, they split evenly all earnings, like Shimon and Azaryah. This connotes that each gets half the reward in Olam ha'Ba, just like they each get half the money earned. This is unlike he wrote in his Perush on the Torah. Also the Chida has contradictory connotations in Rosh David. In Kedoshim, he says only about a Chacham's wife that her share does not detract from his. I wanted to say that when they stipulated, the reward is divided, but when they did not stipulate, each receives the full reward. However, it is hard to say so, for they did not allude to this.

Tzitz Eliezer: Rather, I explain according to the Hafla'ah. The Chacham does not lose his share. Rather, he enables the Am ha'Aretz to receive in Olam ha'Ba the portion destined for him had he engaged in this world in his Torah. Midbar Kedmus (of the Chida) brings this idea, that the supporter gets back his share, from the Yalkut Reuveni in Parshas Re'eh. They will teach to him in Olam ha'Ba. Imrei Binah says that the reward for Mitzvos is double. There is a reward like a Segulah that clings to good deeds. One draws on himself a Divine influence and eternal spiritual life through doing Mitzvos for Hash-m's sake. These are the ornaments and clothing for the Neshamah of a Yisrael. There is also reward from Hash-m, as if people did work for Him. The latter reward can be shared, but the former cannot.

Minchas Yitzchak (6:100): Rav Hai Gaon holds that if the Chacham learns Lo Lishmah, he harms only himself. The one who supports him does not lose. Rav Hai Gaon says that a stipulation cannot work to share the reward. It is not clear whether we can say that he discusses only the first reward.