Bava Kamma 70A

כסתם יחידאה לא אמר.

did not say to apply his principle, that the halakha is always in accordance with an unattributed Mishnah, in a case where the Mishnah expresses the opinion of an individual Sage. According to this interpretation R' Yoḥanan does not rule in accordance with the Mishnah, due to the fact that it represents the opinion of an individual Sage, R' Dosa. Rather, he follows the majority opinion. Yet now that R' Yoḥanan said that the pious ones and R' Dosa said the same thing, it cannot be claimed that they are one and the same Sage. Consequently, the ruling of the Mishnah is evidently accepted by at least two rabbis, and therefore the difficulty from the Mishnah cannot be resolved by assigning it to a single Sage.

אמרי נהרדעי: לא כתבינן אורכתא אמטלטלי. אמר רב אשי לאמימר: מאי טעמא? אמר ליה: משום דרבי יוחנן,

§ The rabbis of Neharde’a say: One cannot write a document of authorization [orakhta] to assign another to collect a debt or a deposit of movable property on his behalf. Rav Ashi said to Ameimar: What is the reason for this halakha? Ameimar said to him: It is due to a principle stated by R' Yoḥanan.

דאמר רבי יוחנן: גזל ולא נתייאשו הבעלים ⁠– שניהם אינן יכולין להקדיש; זה לפי שאינו שלו, וזה לפי שאינו ברשותו.

As R' Yoḥanan says: If one stole an item and the owners have not yet despaired of recovering it, neither of them is able to consecrate it: This one, the thief, cannot consecrate the item because it does not belong to him, and that one, the owner, cannot consecrate it because it is not in his possession. The granting of authority to collect a debt involves the transfer of ownership of the item or money from the owner to the collector; otherwise the debtor could refuse to convey it to the collector. In the case of movable property, as it is being held by another, it is not in the possession of the owner; just as the owner cannot consecrate this item he cannot transfer ownership of it.