Shevuot (Oaths) 43 - Lender is Responsible for Collateral
If one lends his fellow money on collateral, he is considered a paid guardian of this collateral. Therefore, should the collateral be lost or stolen, the lender is liable for it. If the collateral was worth more than the loan, the lender must pay the balance to the borrower, and if it was worth less than the loan, the borrower pays the balance to the lender.
If the borrower claims that the collateral was worth the amount of his loan, and the lender says that it was less, the borrower is free from an oath, since there was no admission of partial liability. However, if the borrower says that he owes a balance, and the lender says that he owes a larger balance, the borrower has to swear.
Who swears as to the value of the collateral? The one in whose possession it was lost, or the lender. If the borrower were to swear and not be careful in assessing the value, and the lender would later produce the deposit, the borrower would be disqualified from taking oaths and from testifying in court. This would be unfair, so the Sages shifted the oath to the lender.
Art: Hans Thoma - Earth