Is the sale valid when a known, undivided portion (musha') of a pile of grain (sabra) or wall is stipulated as an exception (Istithna')?

Primary text

The sale is valid when a known, undivided portion, such as one-third or one-fourth, or specific fractions like two-sevenths or three-eighths, is stipulated as an exception from a pile of grain or a wall. This view is held by the companions of our school (Ashabuna) and is the established position of Imam Al-Shafi'i. The validity stems from the fact that this stipulation does not lead to ambiguity regarding either the excepted part or the part being sold. This is analogous to purchasing a specific tree. The meaning of 'I sold you this pile except for its third' is equivalent to 'I sold you its two-thirds,' and 'except for its fourth' means 'I sold you three-fourths of it.' Consequently, the buyer and seller become partners in the item, with the buyer owning the remaining two-thirds and the seller retaining the excepted one-third.