Gemara

(Rabah): One watching an a Aveidah (lost object) is considered a Shomer Chinam (he is exempt if it gets lost again or is stolen);

This is because he gets no benefit for watching it.

(Rav Yosef): He is like a Shomer Sachar (he is liable for loss or theft).

This is because while caring for the Aveidah he is exempt from giving Tzedakah. Alternatively, it is because the Torah obligated him to watch the Aveidah.

Bava Metzia 30b (Mishnah): If Reuven lost one Sela of earnings while returning an Aveidah, he may not demand full compensation. He is paid like a worker.

If he wants full compensation, if there is a Beis Din there, he stipulates with Beis Din; if not, he is exempt from returning, because he would lose money.

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

Question: He was not idle. He returned an Aveidah!

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

68a (Mishnah): If Shimon gave an Iska to Levi (merchandise which Levi will sell for a profit. Half is a loan, and half is a deposit), he must pay him like a worker.

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

Question: What does this mean?

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

Bechoros 29a (Mishnah): If one was paid for any of the following Mitzvos, the Mitzvah is invalid: to judge, testify, sprinkle or be Mekadesh Mei Chatas (to mix the ashes of Parah Adumah with the water).

If a worker was idle from his job, one may pay him like a worker.

Rishonim

The Rif (Bava Metzia 17a-b and 39b) brings these Gemaros in Bava Metzia

Nimukei Yosef (17b DH Shel): R. Chananel says that even if one returned an Aveidah when there was great demand for labor (it was expensive) and he could have earned a Sela in the time he spent caring for the Aveidah, he does not get a Sela. Rather, he receives based on when there is little work, e.g. a half-Sela. The Rif said the same in a Teshuvah. Rather, we evaluate based on the cheap rate. The Ran says that this is because there was no Beis Din there; he did not need to cease working and lose a Sela. (He could have left the Aveidah there. Since he did not, this shows that he accepted the lower wage.) Therefore, Chachamim imposed this compromise. If so, he should not receive anything, like Mavri'ach Ari (one who prevented damage) is not paid, even for his expenditures! The Ramban answered that it is as if he gave money to the one who lost the Aveidah.

Rambam (Hilchos Gezeilah 12:4): If one was working and interrupted to return an Aveidah worth 100 Sela'im, and lost a Sela of earnings, he does not receive a Sela. Rather, he is paid like an idle worker to be Batel from the job he was doing.

Magid Mishneh: These are the words of the Gemara. The Rashba, R. Chananel, Ra'avad and Rif explain that 'a worker idle from that job' refers to when there is little demand for work, and one would agree to work and not be idle for a small amount. The Rambam explains differently.

Perush ha'Mishnayos (Bechoros 29a, brought in Magid Mishneh 18:14): A strong, adept man who earns much does not receive what he lost. Rather, he is paid based on what standard workers of his profession earn.

Rosh (Bava Metzia 2:24): Rashi explains that we evaluate how much one would want to take to be idle from his job. It depends on how strenuous the job is and how much he is paid. (E.g.) if his job pays four, he would agree to be idle if he were paid one, and he would agree to return an Aveidah if he were paid two.

Rosh (28): Hashavas Aveidah is for free if he was idle. Chachamim enacted to be paid for it in order that one will neglect his job and return the Aveidah.

Rosh (5:39): I explained like Rashi regarding an Aveidah, for there, he should receive his full wage. Regarding Iska, Chachamim were lenient about Ribis mid'Rabanan. If he gives a little (ignores the toil), it does not look like Ribis.

Poskim

Shulchan Aruch (CM 265:1): One who finds an Aveidah must return it for free if he was idle. If he was working and interrupted to return an Aveidah worth 100 Sela'im, and lost a Sela of earnings, he does not receive a Sela. Rather, he is paid like a worker who will be idle and neglect the job that he was doing.

Rema: Some explain how much one would want to take to be idle from his job and engage in returning the Aveidah. E.g. if his job pays four, he would agree to be idle if he were paid one, and he would agree to return an Aveidah if he were paid two, he is paid two, even though half of this is for returning the Aveidah.

SMA (1): The Shulchan Aruch and Rambam do not give any compensation for the toil of returning the Aveidah. The Rema rules like the Tur, who does, because a worker is not obligated to return an Aveidah.

Drishah (YD 177:1): 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. Tosfos asked 'if so, he receives reward for returning.' I.e. since he is compensated for the toil, not just for lost wages, this is reward for the Mitzvah.

Shach (1): This opinion is not primary. Abaye used the same words to teach that he is paid like an idle worker regarding Aveidah and Iska. It is difficult to say that they mean different things. Tosfos distinguished because regarding Iska, the Gemara did not ask that he is not idle. Just the contrary! Why did the Gemara presume regarding Aveidah that he should be paid for his toil? One should not be paid for Hashavas Aveidah! Rather, the Gemara understood that he is paid like one who was idle (before finding the Aveidah), and asked that he was not idle! (The Mishnah said 'if he lost one Sela of earnings...') It answered that we evaluate how much he would want to neglect his job and be totally idle. This is like Bava Kama 56b and Bava Metzia 31a, which say that a Shomer Aveidah gets no reward.

Rema (ibid.): If the money for returning exceeds his normal wage, one pays only his normal wage.

SMA (3): Since he does not lose by returning the Aveidah, he is commanded to do so, and he may not take compensation (above his normal wage).