Siman 187

Any agent who claims that an accident occurred to him and he lost such and such, he swears the oath of the guards and is exempt. An agent who went and acted for free, who is only liable for negligence, must swear that he was not negligent with it, that he did not misappropriate it, and that it was stolen or lost. An agent for hire must swear that it was lost by accident. And if the accident occurred in a place where many people are commonly found, he must bring witnesses and is not believed with an oath, and he pays, as it is written: "no one seeing, the oath of Hashem shall be between them both," and our Sages, of blessed memory, expounded that in a place where there is a seer, one does not fulfill his obligation with an oath, but only with witnesses.

And there was an incident involving one who said to his agent, "Buy me four hundred jars of wine" from his money that was in his hand, and he bought for him, and afterward it was found to be vinegar. The sages said: For such a large amount that soured, there is a report of this and it is possible to bring proof. Therefore, he must bring proof that at the time he took it, the wine was good; and if he does not bring proof, he must pay. And so too in any similar case. But regarding a matter where proof is not commonly found, he takes an oath and is exempt. And so too, one who carries an object in his hand, even in a place where many people are commonly found, and an accident occurred to it, and it is not certain that many would notice the accident, he takes an oath and is exempt [Sma]. And this matter depends on the assessment of the beit din. And regarding this, that the sages required bringing witnesses for the wine, some say that this is when the agent received the wine to himself from the seller; but if he did not receive the wine to himself and left it with the seller so that the buyer would receive it from his domain, it is not relevant to say that there is a report of this, for since he has no guardianship over the wine, he would not put out the report, and he is exempt by means of an oath [Taz]. And it must be said that there was some kinyan beforehand in a manner that the sale is established, and the seller claims that it was good wine and he should not have left it until it soured; for if not so, the kinyan would be void, as money does not effect a kinyan except for "He who punished," and when it is spoiled there is no "He who punished," as I have written in siman 204.

And such is the law regarding a partner who claims that an accident occurred, or regarding other shomrim: if it is a place where many people are present, he must bring witnesses, and if not, he must pay. And even in a case where he is not liable for an oath, such as where he has a migo, nevertheless, in a place where there are onlookers, he must bring witnesses [Netivot HaMishpat]. And if the onlookers are not fit for testimony, such as women, nevertheless it appears that if the claimant argues, "Let these people come and say that it is as you say," his claim is a valid claim, for in the verse it is written, "there is no onlooker," implying that whenever there is an onlooker, whoever they may be, he must bring them. And even if we did not know that he was in a place of onlookers, but the defendant himself says so, nevertheless he is obligated to bring them [ibid.], for a migo does not exempt in this case, as has been explained. And if he says that he cannot bring them, such as if they are in distant places or the like, he is exempted by an oath [as is evident from Tosafot, Bava Batra 170b, s.v. "Ela"]. Our teacher, the Rema, wrote that all this is only regarding shomrim, but regarding a claim of payment or return and the like, as people are prone to pay in private, even if there is an onlooker, he is exempted by an oath. Thus far his words. And if he says of his own accord, "I paid in the presence of witnesses," he must bring them, for he is no better than the borrower himself when he says so, who must clarify, as written in siman 70. And if he brought them and they contradicted him, it has been explained there. And if he told a messenger to buy him something with the messenger's money and he would pay him back, and after a time the messenger claimed that he bought it for him and it was taken from him by force, he shall swear that it is so, and the sender is obligated to pay him, as in the case of a shopkeeper regarding his ledger [Netivot HaMishpat].

Our teacher, the Beit Yosef, wrote: If one gave money to his fellow to buy wheat for him, and he did so and placed the wheat in a certain house and they rotted due to rain that was upon them, he is liable to pay. Thus far his words. For he should have guarded them and seen to it that rain did not fall upon them [Levush]. And likewise, if they were spoiled due to the location, being a place that produces mold, he is a negligent party, and so too anything similar to this. And even if he were a shomer chinam he is liable, for it is negligence. And so ruled Maharik, Shoresh 153, not like the one who wavered in this and says that Maharik was in doubt; it is not so, for Maharik, of blessed memory, ruled so with simplicity, see there.