What is the ruling if a bequest is made of a fraction of an item (e.g., one-third of a slave or house) and the legatee is entitled to a greater fraction?

Primary text

If one-third of a slave or a house is bequeathed, and the legatee is entitled to two-thirds of that item, then the remaining one-third belongs entirely to the legatee. This is held by Al-Shafi'i and the proponents of opinion because the remaining portion is entirely bequeathed, having already been accounted for within the one-third limit, thus belonging to the legatee as if it were a specific item.