Bava Metzia, Chapter 3, Mishnah 11. Our Mishnah discusses the question of when a watchman is permitted to use money given to him for safekeeping, and if he is indeed permitted to do so - what level of guarding is expected of him and in which cases he bears liability.
The Guiding Principle:
In a case where the parties did not explicitly agree whether the watchman is permitted to use the money deposited with him, this is derived from what is standard and accepted, and from the circumstantial evidence before us. The Sages assess the mind of the depositor and determine what a regular person would assume when acting in this manner.
The Mishnah differentiates between three types of watchmen: the shulchani, the homeowner, and the storekeeper.
The Shulchani:
The literal meaning of 'shulchani' is a man of the table, referring to a financier - a moneylender or money changer, who provides financial services in cash. Since his entire business deals with money, it is reasonable to expect that if a person deposited coins with him for safekeeping, he will use them, unless he was explicitly told not to do so, or there is a sign indicating that he is not supposed to do so.
And since he stands to benefit from the money, he is not an unpaid watchman but a paid watchman. His payment is the benefit of the very possibility: the money sits in his drawer, he is open for business, and he can take advantage of a transaction that comes his way. The mere possibility of making a transaction is what renders him a paid watchman, even if he did not actually use the money.
Therefore, from the moment he received the coins and placed them in his drawer, his status is that of a paid watchman - exempt in cases of unavoidable accident, but liable in cases of theft and loss:
If lightning struck his store, or armed robbers came, aimed their weapons at him, and took his money - he is exempt, since a paid watchman is exempt in cases of unavoidable accident.
If thieves came in the night - he is liable, as a paid watchman is liable in a case of theft.
However, if he actually used the money - lent it out, used it for business, or financed something with it - he becomes a full-fledged borrower and is no longer merely a watchman. The law of a borrower of money is like the law of a borrower of an item, and therefore he is liable not only for theft and loss, but even for unavoidable accidents. From the moment he borrows the money, the obligation to return it falls upon him, and it makes no difference if circumstances beyond his control occurred: if lightning struck the person to whom he lent the money, or armed robbers stole it - he is obligated to return it in any case.
It is possible, of course, that the depositor does not want the shulchani to use his money. However, if this is his intention, he must clarify it by securing the money in a way that screams: "Do not open this vessel and do not mix the money with your money." If he did not do so, the understanding is that the shulchani is permitted to use it.
The text of the Mishnah:
"Hamafkid ma'os eitzel shulchani, im tzrurin - lo yishtamesh bahen, lefichach im avdu eino chayav beachrayusan" - Generally, money is kept tied in a sort of cloth, but here it is tied in a way that screams "do not open me": a wax seal, or a special and unusual knot that will immediately give away that someone touched it (nowadays, even a sealed envelope can be considered as such, since it is not meant to be opened). By the very fact that it is closed and sealed in a tamper-evident manner, the message to the watchman is: do not open. And since this is the case, if the money is lost or stolen - the money changer is not obligated to compensate, as his status is that of an unpaid watchman.
"Mutarin - yishtamesh bahen, lefichach im avdu chayav beachrayusan" - The word "mutarin" here means that they are untied, meaning that the money is kept in a cloth in a way that is not particularly secure. Even if it has a slipknot, it can be opened very easily. This is similar to an envelope whose flap is simply folded inside: the money is indeed inside, but access to it is simple, and it is as if it is saying, "here is the money, and if you need it - take it out." The same applies to money placed in an unsealed container. Therefore, he is permitted to use the money, and if it is lost or stolen - he must return the funds, as his status is that of a paid watchman. And as mentioned, all this is before he actually used the money; once he has actually used it, his status becomes that of a borrower, and he is liable even for unavoidable accidents.
The Homeowner:
The second case is one who deposits his money with his friend, an ordinary homeowner, to watch in his house. The Mishnah's assumption is that people do not have much use for cash and do not need to spend it frequently - this was the nature of the economy in the time of the Mishnah, and to some extent, the situation is returning to this nowadays, with the transition to credit cards and digital payment methods. And since the homeowner has no real use for cash, the assumption is that he will not use it.
"Bein tzrurin uvein mutarin - lo yishtamesh bahen, lefichach im avdu eino chayav beachrayusan" - Whether the depositor tied the money in a way that tampering is noticeable or not, it makes no difference. The assumption is that the depositor did not intend for the homeowner to use the money, and therefore he is forbidden to use it. Because of this, if anything happens to the money - even loss or theft, and certainly an unavoidable accident - the homeowner is not required to pay, since he was an unpaid watchman and derived no benefit from the money.
The Storekeeper - The Intermediate Case:
The storekeeper is the owner of the store, and regarding his case, the Tanna'im disagree. The dispute revolves around the question of how much the storekeeper needs cash. The main idea is that in those days, they did not use cash much when dealing with customers: there was a sort of open tab system, and most things were done on credit. And since there is no frequent need for small change, the assumption is that the storekeeper will not have a use for the money deposited in his hands.
On the other hand, the storekeeper needs cash to buy his merchandise: when he comes across good merchandise at a cheap price, he will certainly take advantage of the opportunity. It turns out that the storekeeper uses money occasionally - more than the homeowner and less than the money changer. And this is the question: how should he be treated?
"Chenvani keva'al habayit, divrei Rabbi Meir" - According to Rabbi Meir, the status of the storekeeper is like that of the homeowner, and he has no permission whatsoever to use the money deposited with him, even if it is not sealed in a container that indicates it was opened.
"Rabbi Yehudah omer: Chenvani keshulchani" - According to Rabbi Yehudah, the storekeeper is expected to use the money, since he can use it for a transaction and buy cheap merchandise if the opportunity arises. Therefore, unless the money was sealed in a way that screams "do not open me," his status is that of a paid watchman and he is liable for theft and loss.
And the halachah follows Rabbi Yehudah: the storekeeper is indeed expected to use the money, and therefore his status is that of a paid watchman.
In summary: Our Mishnah establishes that the permission to use deposited funds is determined by the presumed intent of the depositor and the typical practice of the watchman. The money changer, whose entire business is money, is allowed to use it when it is not bound, and therefore his status is that of a paid watchman; and if he actually uses it, he becomes a borrower and is liable even for unavoidable accidents. The homeowner, who has no use for cash, is not allowed to use the money at all, and his status is that of an unpaid watchman. Regarding the storekeeper, Rabbi Meir and Rabbi Yehudah disagree, and the halachah follows Rabbi Yehudah that his status is like that of the money changer.