Bava Batra 87 - Selling Wine or Oil

If one was selling wine or oil to his fellow, and after the price was fixed, the merchandise appreciated or depreciated, who stands to gains or lose?

If the price changed before the measure was full, it changed for the seller. If it rose, the seller can retract and insist on a higher price. If it fell, the buyer can retract and hold out for a lower price. Once the measure is filled, the buyer acquires the merchandise and the deal is final.

But how can this ruling be true? If the container belongs to the seller, he owns the merchandise until the buyer takes delivery. And if it belongs to the buyer, he acquires every measure as it is poured in. The answer is that the container belongs to a middleman, who loans it to the seller while filling, then loans it to the buyer after it's full.