Is the surety (daman) valid for an unknown quantity of liability?
Chapter on Guarantee (Daman)
Al-Mughni
Book of Assignment (Transfer of Debt)
Primary text
The surety for an unknown amount is valid. This is supported by the statement, 'Whatever I give to you is upon me,' where the amount is unspecified. This view is held by Abu Hanifa and Malik. The evidence relies upon the statement of Allah the Almighty: {And whoever brings a camel-load [of goods], he will have [a reward] and I am responsible for it} (Quran 12:72), as the camel-load is variable and unknown. Also supporting this is the general statement of the Prophet (peace be upon him): "The guarantor is liable" (al-za'im gharim), and because it is an undertaking of a right upon one's personal liability without compensation, it is valid for the unknown, similar to a vow (nadhr) or confession (iqrar). Furthermore, it is valid even when suspended upon a risk or danger, such as guaranteeing an obligation.
Supporting text
Al-Thawri, Al-Layth, Ibn Abi Layla, Al-Shafi'i, and Ibn al-Mundhir rule that it is not valid because it is an assumption of a financial obligation, which cannot be validly undertaken if unknown, analogous to the price in a sale.