Bava Kamma 52A

כיון שמסר לו מפתח ⁠– קנה.

once he has conveyed the key to him, he has acquired it.

היכי דמי? אי בכספא ⁠– ליקני בכספא! אי בחזקה ⁠– ליקני בחזקה! לעולם בחזקה, ובעי למימר ליה ״לך חזק וקני״; וכיון שמסר לו מפתח, כמאן דאמר ליה ״לך חזק וקני״ דמי.

Here too, the Talmud asks: What are the circumstances involving this sale? If it was a transaction by payment of money, then let him acquire it by paying money. If it was by taking possession of it, let him acquire it by taking possession. What is the significance of transferring the key? The Talmud answers: Actually, the transaction occurred by taking possession, and in that case, the seller usually must say to him: Go, take possession, and thereby acquire it for yourself. And in this case, once he conveys the key to him, he is considered like one who says to him: Go, take possession, and thereby acquire it.

אמר ריש לקיש משום רבי ינאי: המוכר עדר לחבירו, כיון שמסר לו משכוכית ⁠– קנה.

Similarly, Reish Lakish says in the name of R' Yannai: With regard to one who sells a flock of sheep to another, once he conveys the mashkukhit to the buyer, he has acquired the flock.

היכי דמי? אי במשיכה ⁠– ליקני במשיכה! אי במסירה ⁠– ליקני במסירה! לעולם במשיכה, ובעי למימר ליה: ״לך משוך וקני״, וכיון דמסר לו משכוכית ⁠– כמאן דאמר ליה: ״לך משוך וקני״ דמי.

The Talmud asks: What are the circumstances of this case? If the transaction occurred by pulling the flock into his possession, then let the buyer acquire it by pulling. If it was by conveying it, let the buyer acquire it by the seller conveying it. The Talmud answers: Actually, it was a transaction by pulling, and in that case, the seller usually must say to him: Go, pull it and acquire it for yourself. And in this case, once he conveys the mashkukhit to him, he is like one who says to him: Go, pull it and acquire it.