Before us is Mishnah 29 in the second chapter of Tractate Bava Metzia, which contains two parts. The first part teaches that the finder must apply his common sense: an animal whose owner appears unaware of its location is considered a lost object and must be returned; whereas an animal whose owner presumably knows where it is requires no involvement at all.
"Eizo hi aveidah" - What is considered a lost object:
The Mishnah clarifies what type of animal is considered lost, contrasting two different scenarios:
"Matza chamor o parah ro'in baderech - ein zo aveidah" - If he found a donkey or a cow grazing on the way, it is not a lost object. An animal grazing by the side of the road is not considered lost. We do not assume it has strayed from its path; rather, its owner presumably knows where it is, and one should not get involved.
"Chamor vekeilav hafuchin" - A donkey and its gear are inverted. This refers to a donkey whose saddle has flipped and is now under its belly instead of on its back. This is a sign that the donkey is unattended and no one is watching it, so the owner presumably does not know where it is.
"Parah ratzah bein hakeramim" - A cow running among the vineyards. A cow wandering inside a vineyard is in a dangerous situation and may get injured. The same applies to a cow fleeing from the city toward the fields.
In the latter two cases, the Mishnah rules: "Harei zo aveidah" - This is a lost object. It is clear that the owner does not know what is happening, for he would not let his donkey or his cow get into such a situation, and therefore the obligation to return it applies.
The obligation to return it again and again:
This is the first ruling. The second ruling states that there is no limit to the number of times the obligation to return it applies: "Hechezirah uvarechah, hechezirah uvarechah, afilu arba vechamesh pe'amim - chayav lehachazirah" - If he returned it and it ran away, returned it and it ran away, even four or five times, he is obligated to return it. The number "four or five" is not limiting; rather, this is the standard style of the Mishnah's language, and the obligation exists for any number of times.
Seemingly, there would be room to exempt the finder and say that the owner is negligent and careless, since he does not lock up his animal and it repeatedly runs away. Therefore, the Mishnah brings the source for the ruling: "Shene'emar: hashev teshivem" - As it is stated: "You shall surely return them." This is a double expression, and from this repetition we learn that the obligation to return it applies even many times.
The second part: Compensation for lost work:
The novelty of the second part is that the Torah does not require a person to lose money in order to return a lost object to his fellow. A person who is free from his work, after working hours, who comes across a lost object must go and return it. But someone who is in the middle of his work is entitled to compensation for the loss of income caused by interrupting his work for the sake of returning the item.
The Mishnah says: "Hayah batel misela" - If he lost a sela of work. For example, a laborer in construction or fieldwork, since during the time of the Mishnah most people engaged in hard and strenuous physical labor. This is a person who lost a wage of a sela, a fair and handsome wage, but in return for it, very hard work is required. It makes sense that he would be happy to take a break from his toil and engage in an easier task for a lower wage. For this reason, he does not receive his full wage as if he had continued digging trenches, for he is not digging; rather, he is engaging in a much easier task that is even comfortable for him. Therefore, his wage is set as the wage of a "po'el batel", like a person who is not working.
Two approaches to understanding "po'el batel":
The view of the Shulchan Aruch: This refers to the wages of a literal idle worker - someone sitting idly, like a watchman who stands and opens a gate. This is the lowest possible wage, since he is not doing any physical labor at all.
The view of the Rema: This refers to the amount a person would agree to receive for the specific type of work he is currently leaving. For ten dollars an hour, a person might agree to sit at the entrance and open and close the door; he would demand twenty dollars an hour for digging trenches and laying bricks. Returning a lost item is somewhere in the middle: the returner is not sitting idle, but he is also not sweating in the sun. Therefore, his wage is like the wage of a messenger - say, twelve dollars an hour. According to the Rema, the law of an "idle worker" means that he does not receive his full wage for his hard labor, but rather the wage that any person would receive for an errand of this nature.
Making a stipulation before a beis din:
What is the halachah when the worker claims: It is true that messengers earn twelve dollars an hour, but I earn twenty dollars an hour laying bricks, and I cannot afford this break, as I have a family to support and I need my full wage. Since the Torah does not require a person to suffer a financial loss against his will, he can make an explicit stipulation - with the loser of the item or before a beis din, even an ad hoc beis din of three men - and say: I am willing to give up an hour of work, but my wage is twenty dollars an hour and I must receive my full wage. If the beis din approves this or the loser agrees to it, he is entitled to his full wage.
As the Mishnah states: "Im yeish sham beis din - masneh lifnei beis din" - if there is a beis din there, he stipulates before the beis din. Since he stipulated in advance, he receives a full refund of the twenty dollars he lost due to his leaving to return the lost item.
"Ve'im ein sham beis din - bifnei mi yasneh? Shelo kodem" - and if there is no beis din there, before whom should he stipulate? His own comes first:
The Mishnah asks rhetorically: when there is no one before whom to stipulate, with whom should he stipulate? The answer is: "Shelo kodem" - his own comes first. The financial considerations of the finder take precedence, and the Torah does not require him to lose money for the benefit of the loser. Therefore, when he cannot make a stipulation and guarantee his full wage, he is permitted to say: I am engaged in my work and cannot take time off for this lost item, let someone else do it. He is exempt from returning it, because his own financial matters come first, and the Torah does not demand a financial loss from him in order to fulfill the mitzvah of returning a lost item.
In summary: In the first part of the Mishnah, we learned the parameters for identifying a lost item - an animal grazing by the way is not a lost item, whereas a donkey whose gear is overturned or a cow running among the vineyards is a lost item, and that the obligation to return it applies repeatedly without limit, as derived from "hasheiv teshiveim." In the second part, we learned that the Torah does not require a financial loss: the returner receives the wage of an "idle worker" - the explanation of which is a dispute between the Shulchan Aruch and the Rema - and he can stipulate before a beis din for his full wage, and when there is no one before whom to stipulate, "his own comes first."