If the borrower claims that the collateral was worth the loan amount, 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: Earth by Hans Thoma