סִימָן קפח – הִלְכוֹת פִּקָּדוֹן
188: Laws of Deposits
Someone who deposits money by a friend, now in these days that all our jobs are in buying and selling and we all need cash, the norm is that the depositor is willing to have the person responsible will take it out [for his own use] when he needs it. Therefore, he may take the deposit out, and it is with him like a loan — unless the depositor revealed that he did not want it. Such as if he sealed it in, or tied them with a unique knot; then the responsible party may not take it out [for his own use].
Someone who deposits another item [as opposed to the money discussed in the previous se'if], the responsible party may not use this item for his own needs. Even if the item would not get damaged at all by this use, in any case, it would be [an instance of] borrowing without [the owner's] knowledge. One who borrows without [their] knowledge is a thief. But if he knows for sure that the depositor doesn’t care about it — it is permitted. Although some prohibit even in this [case], since a deposit — even of something people don’t normally deposit — is prohibited, because one who sends a hand to a deposit also similarly [a thief]. And there is [good reason] to be stringent.