Sefer HaChinuch 57 ספר החינוך נ״ז

The commandment on the court to judge the case of an unpaid guardian: To judge the case of an unpaid guardian, as it is stated (Exodus 22:6), “If a man gives his neighbor silver or vessels to keep, etc.” And the explanation that comes for it (Bava Metzia 94b) is that this section is stated about an unpaid guardian. And therefore it exempted him from [liability for] theft. And the understanding of “unpaid” is that the [guardian] did not receive any wage for his guardianship from the [owner].

The root is well-known. Its laws — for example, what is the law of one who makes the claim of theft about the deposited item and swears [that it is so] and afterwards witnesses come [and testify] that it is on his property, and he goes back and [again] makes the claim of theft and swears and afterwards witnesses come that it is on his property (Bava Metzia 108a); the law of one who makes the claim of [it being] lost; what is the law of one who makes a claim of it being deposited; the law of one who makes a claim and goes back and makes a claim of theft and afterwards witnesses come that it is on his property; the law of one who makes the claim of theft about the deposited item of a minor; and the rest of its details — are elucidated in the ninth chapter of [Bava] Kamma and the third of [Bava] Metzia and the eighth of Shevuot. (See Tur, Choshen Mishpat 291.)

And [it] is practiced in every place and at all times. And a court that transgresses it has violated a positive commandment.