why would mixing be prohibited? why would rights to mixing be forfeited? if a buyer is buying impure oil why can't the seller give that to him even if the oil got separated (purified)?
israel, Beit Shemesh
The Ritva offers two explanations. The opinion which forbids mixing oil refers to when they spoke of selling "this" oil, so even though no mention was made of purity, he cannot tamper with the oil.
Another answer is that the seller has no impure oil at all. He is not allowed to supply impure oil from someone else since the buyer expects to receive from what the seller has.
(According to the Shulchan Aruch (Choshen Mishpat 228:15), one may not mix dregs from a different barrel, but he may mix from the same barrel. This, though, depends on the Rosh and Rambam on this Daf.)
All the best,
Reuven Weiner