Reference for Bava Metzia 38:6

כי קתני בברייתא אע"פ ששניהם מודים לא יחזיר לא לזה ולא לזה בבריא דלאו בר מהדר הוא

because [the donor] withdrew it from the former. But the Baraitha, which teaches that even if both parties admit [the validity of the found document] it shall not be returned to either party, deals with a healthy person, who cannot withdraw,<span class="x" onmousemove="('comment',' He cannot change his mind after he has made a gift to a person and handed him the document conferring the gift. ');">⁶</span>

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