Is a sale valid if a specific quantity is stated from an unmeasured, known heap (Ṣabrah)?

Primary text

The sale is valid if the seller states, "I sold you one qafiz (measure) from this heap," or "ten qafiz," provided both parties know the heap contains a quantity greater than that stated amount. The basis for validity is that the sold item is a determinable, known quantity extracted from an aggregate unit whose sale is permissible. This is analogous to selling half of the heap. By witnessing the entire heap, the buyer has witnessed the object of the sale, as it is a portion thereof.