Gemara

30b (Mishnah): If in the time Reuven spent returning an Aveidah, he lost one Sela of earnings, he may not demand compensation of one Sela. Rather, he is paid like a worker.

31b (Beraisa): He is paid like an idle worker.

Question: He did not rest. He returned an Aveidah!

Answer (Abaye): He is paid like a worker idle from that job.

68a (Mishnah): If Shimon gave goods to Reuven to sell (for a higher price. Half the goods are a loan, half are a deposit. Each gets the profit on his half;) this is permitted only if in addition he pays him like a worker (for selling Shimon's half).

The same applies if he gave money to buy goods and share the profit.

(Beraisa): He is paid like an idle worker.

Question: What does this mean?

Answer (Abaye): He is paid like a worker idle from the job he was engaging in.

The Mishnah needed to teach both cases.

Had it taught only about selling in a store, one might have thought that since it is light work, he is paid only like an idle worker, but to buy merchandise, he would get his full normal wage;

Had it taught only about buying merchandise, one might have thought that since it is hard work, he is paid like an idle worker, but to sell in a store, he gets a pittance, and even if he ate a drop of Shimon's food, that is enough.

The Mishnah teaches that this is not so.

(Beraisa) Question: How much is he paid?

Answer #1 (R. Meir): (Whatever they stipulate suffices,) be it much or little;

Answer #2 (R. Yehudah): It suffices if he eats vegetables dipped in brine or a dried fig of Reuven;

Answer #3 (R. Shimon): He must receive his full normal wage.

Rishonim

Rif (39b): The Halachah follows R. Meir (who says that whatever they stipulate suffices), for the Stam Mishnah is like him. Every day of the partnership, he must give like a worker idle from that job. It says 'be it much or little', according to what they stipulated. Alternatively, they may fix a wage for the entire partnership. Rava (69a) taught that if the investor gets half the profit, he must bear two thirds of the loss. If the investor bears half the loss, he gets only a third of the profit. The extra profit for the worker is his wage.

Rebuttal (Ba'al ha'Ma'or): The Halachah follows R. Shimon (who says that he gets his full normal wage), for the Stam Mishnah is like him. One who hired workers and did not have work for them pays their full wage, except he deducts for there was less toil. This is only for a grocer, who engages in the Iska the entire day, but not for one who raises another's animals.

Defense (Milchamos Hash-m): In a Teshuvah, the Rif brought three proofs that are Mishnah is R. Meir. 1. A Stam Mishnah is assumes to be R. Meir. If it were like R. Shimon, the Gemara would have said so. 2. In a Tosefta, R. Meir says that he is paid like an idle worker. 3. The Gemara brought the Tosefta that says 'he is paid like an idle worker' to explain our Mishnah. This explains the Mishnah only if it is like R. Meir; who said 'be it much or little', i.e. whatever they stipulated. Also, the Gemara discusses R. Meir's opinion. This shows that the Halachah follows him.

Rambam (Hilchos Shutafim 6:2): Chachamim enacted that if Reuven does business with Shimon's money, half is a loan and half is a deposit. Each has Acharayos, i.e. suffers losses, even b'Ones, on his half. We cannot say that each gets half the profits. If so Reuven works in exchange for the loan, and this is Avak Ribis. If they want to share the profits, Shimon must pay Reuven for his toil for every day of the partnership, like a worker idle from that the job that he neglects.

Rosh (5:39): Abaye explained that he gets like one idle from that job. I.e., regardless of his job, he is paid like one who had no job. This is like R. Meir. Rashi explained 'like one idle from that job', i.e. how much one would want to be paid to do this easy task, in place of his normal wage for his more strenuous job. If so, our Mishnah is like R. Shimon. A Tosefta says that it is like R. Meir! Also, why did the Gemara suggest that pay like an idle worker would not suffice for buying merchandise, which is a toil? He is paid according to his toil! Even though I explained like Rashi above regarding an Aveidah, here is different. There, he should receive his full wage. Here, Chachamim were lenient about Ribis mid'Rabanan. If he gives a little, it does not look like Ribis.

Poskim

Shulchan Aruch (YD 177:2): If Reuven does Iska with Shimon's merchandise, and half the Acharayos is Reuven's and half is Shimon's, it is forbidden. Iska is half a loan and half a deposit. Reuven toils for the deposit in exchange for the loan. Therefore, Shimon must pay Reuven for his toil for every day of the partnership, like a worker idle from the job that he neglects.

Tur: He is paid the amount that one without a job would want to do such toil.

Rebuttal (Beis Yosef DH v'Lo Kol): This is unlike the Rosh and Tosfos. They rejected Rashi, because the Gemara asked why we ignore his toil for the Iska (and according to Rashi, we do not ignore it)! This is difficult also for the Tur! Rather, he is paid like one who is totally idle. Tosfos ha'Rosh also says so. The Rif holds like Rashi. The Mishnah Torah cites the words of the Gemara. This connotes like the Rif. Perush ha'Mishnayos is like Rashi.

Defense (Drishah 1, citing Chachmas Shlomo): Someone without a job would not demand very much. The Gemara asked that it is unreasonable to be so lenient when engaging in the Iska requires much toil. Even though he is paid for the toil, it is not very much. The Beis Yosef understood that we evaluate how much one he would want to take to be idle from his job and do nothing. This is wrong. Regarding Hashavas Aveidah, the Gemara asked that 'idle' connotes that he is totally idle. Why didn't the Gemara ask also here (regarding Ribis) that he is not totally idle?!

Drishah (1): Tosfos explains like the Beis Yosef. Regarding Hashavas Aveidah, Tosfos explains like Rashi. The questioner thought that we evaluate how much he would want to be totally idle. Abaye answered that we evaluate how much he would want to do the easier task. Regarding Iska, we asked what 'like an idle worker' means, and answered like the questioner thought regarding Hashavas Aveidah. Here, the Gemara did not ask that he is not totally idle, for it knew that the borrower would happily work without extra compensation. Chachamim required something lest it look like Avak Ribis, so any amount suffices. Also, regarding Hashavas Aveidah the Gemara did not ask that he is not totally idle. Abaye 'asked', in order to give his answer!

Shach (5): Some Poskim say that we always consider how much one without a job would want, even if Reuven had a lucrative job. The Beis Yosef holds like Poskim who say that we evaluate how much he would want to be totally Batel, regardless of the task he will need to do. RashI say that we evaluate how much he would want to cease from his higher paying job, which is more strenuous. The Rashba says that we consider how much he earns when his profession is in low demand.