Here we'll talk about the exact completion of a sale and when neither party has the ability to withdraw from it. We'll talk about a person filling a container with liquid. The seller is selling liquid, oil or wine to the buyer, and he's using a container belonging to a third party. That's an important detail here, because if it were the buyer's container, the moment everything starts going into it, he acquires it right away. So here there's a third-party container. We consider the container that belongs to somebody else to be the seller's until the container is fully filled up. It's on loan by the seller. He's using it. Once it's filled, it's on loan to the buyer because he's supposed to transport it home.
The Mishnah says: Somebody's selling wine or oil to his friend. As the liquid is being poured into a borrowed container, the price either goes up or goes down. Either way, one of the parties wants to withdraw because he's getting the raw end of the deal. So the halacha (Jewish law) is as follows: If the price changed before the container was completely filled, then the seller will have to sell the liquid. All of the wine and oil still belong to the seller, and therefore either party is still allowed to withdraw from the sale. It's owned by the mocher (seller), and therefore the deal hasn't been completed yet. But if the price changed after the container was filled, the goods have been transferred to the buyer. Once that happened, the buyer automatically owns the goods, and the sale is considered complete. They can't back out.
Now we'll talk about another variable. What if they're not dealing directly with each other, but there's a broker in between them? The broker here is a middleman. He's not a shaliach. He's not an agent of the buyer. Rather, he's a middleman working between them. He's going to take temporary possession of the item after it's given to him by the seller, until he hands it over to the buyer.
If there was a broker between them, the container here belongs to the broker. He's buying wine or oil from the seller and then selling it to the buyer. If the barrel breaks in the interim, while it's being held by the broker after it's filled, then the broker is the one who loses out. The seller has already transferred ownership of it to the broker. The broker took possession of it, and then he's planning on selling it to the buyer. But the buyer hasn't taken possession yet. Therefore, the one who loses out here is the broker.
Other laws apply to liquid. The seller has to tip out three drops for the buyer. If somebody's selling a whole container of wine or oil, what he has to do first is empty the entire container. But even after it's empty, he has to allow the extra liquid still in there to drip out—three drops into the buyer's container. That belongs to the buyer. Once those three drops have been given to the buyer, the seller tilts the container on its side. If more liquid gathers there, that still belongs to the seller. When the buyer expected to purchase the container, he didn't expect to receive every last drop that was in it. So if there's a little bit left over, that still belongs to the seller.
The shopkeeper, who's making sales one after another, is busy and in action. He doesn't have to tip out the last three drops for each buyer. He's busy, and therefore we're not going to trouble him to go through all of this for the extra few drops.
According to Rabbi Yehuda here, that exemption for the shopkeeper is limited. He says that on Friday, when it's very close to Shabbat, the shopkeeper is exempt because he's very busy. According to Rabbi Yehuda, this whole idea that the shopkeeper is exempt from giving those three extra drops because he's very busy and very rushed only applies right before Shabbat. He doesn't have to give them because of the chaos happening on Erev Shabbat (right before Shabbat). At all other times, the shopkeeper does have to tip out those three drops to the buyer.