What is the normal basis for loss of a watch securing a debt of 100 dirhams?

Kitab al-Rahn: Salam, Sarf, Third-party Custody and Fungible Collateral

• The count of dirhams equals the watch’s value
• The watch always discharges all 100 dirhams
• The lesser of secured watch value and the debt
• Treating the watch’s weight as a coin count