Rambam - 1 Chapter a Day

Without consulting the borrower, as reflected in the continuation of the halachah.

I.e., the son of the owner.

The commentaries question: How can the person become the borrower’s agent without being directly appointed to serve in that capacity? And conversely, if he was directly appointed in that capacity, why is the borrower not liable? Among the explanations given is that we are not speaking about a person actually appointed as an agent by the borrower, but about one who volunteered to serve in that capacity on his own initiative.

This law applies both to a Hebrew servant and a Canaanite servant. Rav Kapach states that in the authentic Yemenite manuscripts of the Mishneh Torah [and in the Rambam’s text of the Mishnah (Bava Metzia 98b), which is the source for this Halachah], “his servant” is not mentioned in this clause.

For the borrower has not accepted responsibility for it yet.

In these instances as well, the borrower did not tell the owner “Send it with this person,” nor did the owner notify the borrower, and the borrower consent.

Since the borrower specifically instructed the owner to send it with the designated person - or agreed to the owner’s suggestion - it is considered as if it entered the borrower’s domain from the time it is handed over to that person.

Only then is there a kinyan that causes the borrower to accept responsibility.
Rabbenu Asher rules that the borrower is liable in such a situation, since the owner followed his instructions. The Shulchan Aruch (Choshen Mishpat 340:7) quotes the Rambam’s ruling, and the Ramah does not object.

Without the owner’s consent.

By consenting, the owner agrees that the article is considered to be returned when given to the person designated.

For the borrower no longer has the right to make use of the article. This change in status applies even if the animal is still in the home of the person who had borrowed it.

He is placed in this category because he derived benefit from having the animal in his possession (Seifer Me’irat Einayim 340:13).

And the borrower is thus liable to make restitution for the animal. A renter, by contrast, is not liable if the animal he is renting dies.

If, however, he denies the owner’s claim different rules apply, as the Rambam continues to explain.

The owner must bring witnesses who testify to the truth of the owner's claim.

The Lechem Mishneh and others raise questions concerning the Rambam’s ruling, based on his statements in Hilchot To’en V’Nit’an 1:7 that a person is never required to take an oath when the plaintiff does not have a definite claim against him. In some of the situations mentioned above, the plaintiff does not have a definite claim. Hence, it would seem appropriate that the watchman not be required to take an oath.
The Lechem Mishneh explains that the principle stated in Hilchot To’en V’Nit’an applies with regard to loans and other claims of that nature. It does not apply to claims concerning borrowed objects, because the owner does not accompany the borrower at all times. Thus, it is likely that we will not be able to lodge a definite claim regarding the loss of the article.
The Shulchan Aruch (Choshen Mishpat 344:4) differs and obligates an oath only when the plaintiff has a definite claim. If he has an indefinite claim, the watchman is obligated only to take an oath that the animal died in an ordinary manner. He must, however, include in that oath that it was the rented ox that died or that he does not know which ox died, because of the convention of gilgul sh’vuah.
All the oaths mentioned by the Rambam in this Halachah are sh’vuot hesset, Rabbinic oaths administered without requiring the defendant to hold a sacred article (Kessef Mishneh).

As long as the watchman does not admit his liability, even if he says that he does not know whether or not he is liable, he is not required to make restitution.
The commentaries question the Rambam’s ruling, asking why the watchman is absolved of responsibility. Seemingly, the question resembles a case in which a person admits taking a loan and when asked to pay states: “I don’t know whether I repaid the debt or not.” In such an instance, as the Rambam rules in Hilchot To’en V’Nit’an 1:9, the borrower is liable. On the surface, the same law should apply in this instance, since the borrower admits accepting responsibility for the cow as a borrower and does not know whether he is absolved of that responsibility or not.
The commentaries attempt to resolve this question, focusing on the concept mentioned by the Kessef Mishneh in his gloss on Chapter 1, Halachah 5. There the Kessef Mishneh explains that although the lien on the borrower’s property begins when the article is taken, the actual obligation does not start until the article is destroyed. Therefore, in this instance, it is as if the borrower does not know whether or not he ever became obligated.

As explained in Hilchot To’en V’Nit’an 1:12, whenever a person is required to take an oath by a plaintiff, the plaintiff may require him to deny any other claims he has against him in that same oath.

The Rambam’s ruling can be explained as follows: In this instance - in contrast to the situation mentioned in the previous Halachah - the watchman is required to take a Scriptural oath, for he is modeh bimiktsat. (He admits a portion of the plaintiff’s claim. The plaintiff claims that he owes him for two cows and he admits owing him for one.) A defendant may not take a Scriptural oath when he does not know whether or not he is liable. Instead, since he cannot take an oath to deny the plaintiff’s claim - he cannot because he does not know - he is required to make financial restitution. (See Hilchot To’en V’Nit’an 4:7-8.)
On this basis, the Maggid Mishneh questions the Rambam’s ruling in the previous Halachah. In that instance, the watchman is required to take a Scriptural oath - as are all watchmen - that the animal died in an ordinary manner. If the watchman claims not to know which of the oxen died- seemingly, because of the convention of gilgul sh’vuah the plaintiff could require him to support his claim with an oath. And since he is unable to take this oath - for he does not know - he should be required to make financial restitution.
The Maggid Mishneh explains that since the oath required of a watchman does not stem from a definite claim, it is not powerful enough to serve as the basis to require an oath through the convention of gilgul sh’vuah.
In his Kessef Mishneh, Rav Yosef Karo offers a different resolution, explaining that the principle that a person who cannot take an oath is required to pay applies only with regard to the fundamental claim concerning which the person is required to take an oath. With regard to a secondary claim for which he is required to take an oath because of the principle of gilgul sh’vuah, it is acceptable to answer: “I don’t know.”
Rav Karo quotes this principle in his Shulchan Aruch (Choshen Mishpat 344:4). Seifer Me’irat Einayim 344:14 and the Siftei Cohen 344:5 explain that the Maggid Mishneh’s explanation has to be coupled with that of the Kessef Mishneh. Otherwise, there would be a contradiction between the Shulchan Aruch’s rulings, for in Choshen Mishpat 75:15, the Shulchan Aruch rules that even in a situation involving gilgul sh’vuah, we follow the principle that a person who cannot take an oath must make restitution.

Hilchot To’en V’Nit’an 4:7-8.