Yosef owned a small Shas that he had bought six years ago, when he first entered yeshiva. It was in good condition, but it was an old printing. When Yosef got engaged, his chevrusa bought him another Shas with a new printing.
“I’ve wanted the new edition for a while,” said Yosef appreciatively. “What can I do with the old Shas?”
“You can leave it for sale on the table outside the beis midrash,” said his chevrusa. “Maybe someone will buy it.”
Yosef put the Shas on the table with a note: “Shas for sale. $30 – Please give money to Yosef or leave in the envelope at his table.”
Two days later, Moshe met Yosef at supper and said: “I’d like to buy your Shas, but I have to run home now.”
“Fine!” said Yosef. “It’s outside the beis midrash; take it when you want.”
“I don’t have money with me,” said Moshe. “Can I pay you tomorrow?”
“No problem,” said Yosef. “It’s yours.”
Meanwhile, Shalom saw the Shas that evening and decided to buy it. He took the Shas and put it on his table. He placed $30 in the envelope and wrote his name on it.
When Moshe came in the morning, he saw the Shas was gone. “Someone took the Shas,” he said to Yosef.
“I know, Shalom bought it,” said Yosef. “When I came this morning there was money in the envelope.”
Moshe went over to Shalom. “When did you buy the Shas?” he asked.
“Last night about 10,” Shalom said.
“I had already spoken with Yosef at supper and he agreed to sell the Shas to me,” said Moshe. “It’s mine.”
“But I took it first,” said Shalom, “so it’s mine!”
Yosef heard them arguing, each one claiming ownership. “Why don’t we ask Rabbi Dayan?” he suggested.
The three went to Rabbi Dayan. “Whom does the Shas belong to?” asked Yosef.
“If, in fact, you had already agreed to sell the Shas to Moshe, Shalom has no claim to it; you can award it to whomever you want, with a preference to Moshe,” answered Rabbi Dayan. “However, if Shalom does not trust that you had already agreed to sell and there is no proof, he can keep the Shas.”
Both Shalom and Moshe asked for an explanation.
“When Yosef initially put the Shas for sale, he expressed willingness to sell it to anyone,” explained Rabbi Dayan. “However, when he agreed to sell it to Moshe, he was no longer interested in selling it to anyone else. Shalom cannot acquire it without Yosef’s consent. Although the Shas remained on the table with a ‘for sale’ note, it no longer expresses intent to sell.”
“If so, shouldn’t the Shas be mine?” asked Moshe. “Yosef agreed to sell it to me at supper!”
“Halacha requires an act of kinyan to confirm a transaction,” replied Rabbi Dayan. “Agreement alone to sell does not finalize it. Thus, you also didn’t acquire the Shas, so Yosef can grant it to Shalom. However, it is unethical and untrustworthy to retract from an agreement, so it is preferable that he grant it to you.” (C.M. 189:1; 204:11)
“What if Shalom does not trust that I had already agreed to sell?” asked Yosef.
“He can claim he acquired the Shas while it was still for sale by picking it up,” said Rabbi Dayan. “He is now in possession, since it is in hands, and we would apply the rule of hamotzi meichaveiro alav har’aya – the burden of proof is on the plaintiff. You are not believed without proof to say that you retracted from your intent to sell, to take the Shas back from Shalom or grant it to Moshe.” (C.M. 222:1,4)