What is the ruling if the intent behind the conditional addition/deduction was related to price adjustment, such as 'I will not charge you the price of one qafiz'?
Chapter on Selling Assets and Fruits
Al-Mughni
Book of Sales
Primary text
If the intent was to waive the price of one qafiz without specifying addition or subtraction of the commodity itself, the ruling depends on the prior knowledge of the heap's quantity. If the total number of qafizes in the heap was known to both parties (e.g., 'This is ten qafizes, I sold them to you, every qafiz for one dirham, on the condition that I add one qafiz from this other heap,' or described uniquely), the sale is valid. This is because it means selling 'every qafiz plus ten qafizes for one dirham.' Similarly, if the quantity of the heap was known and the intent was not to charge for one qafiz, the sale is valid because the deduction from the total price can be calculated.
Supporting text
If the specific unit (qafiz) is unknown or if the waived amount was stipulated as a gift ('hibah'), the sale is invalid. If the condition was specified as a deduction ('on the condition that I deduct one qafiz'), it is valid, meaning the sale is for nine qafizes for ten dirhams, making every qafiz priced at one dirham and one-ninth.