Is a sale valid if the seller says: 'I sold you this heap for ten dirhams, on the condition that I add one qafiz to you, or deduct one qafiz'?
Chapter on Selling Assets and Fruits
Al-Mughni
Book of Sales
Primary text
The sale is invalid because it is unknown whether the seller intends to add or deduct. If the seller specifies only addition ('on the condition that I add one qafiz'), the sale is void because the specific unit (qafiz) is undefined/unknown.
Supporting text
If the seller states, 'on the condition that I add one qafiz from this other heap,' or describes it with a definitive attribute, the sale is valid. This means the contract is for the stated heap plus one qafiz from the other heap for ten dirhams.