Should the old or new value be calculated for something that was lent, as long as there is a significant difference between them?

Lending is the transfer of usufruct without compensation. If the lent item remains with the borrower, the lender owns the item itself. If the lender desires its price, then that is a sale they initiate at a price they agree upon.

However, if the lent item perishes, the borrower guarantees it if its perishing was due to transgression or negligence on their part. If it perishes without transgression or negligence, there is a difference of opinion; however, the Shafi'is and Hanbalis hold that the borrower guarantees it, unless it was damaged during authorized use, in which case nothing is guaranteed.

If the item is damaged through its use in a manner not authorized, the borrower guarantees it. The value is assessed at the time of damage.