Reference for Arakhin 59:3

דלמא אינה מכורה ומעות מתנה מידי דהוה אמקדש אחותו דאיתמר

Perhaps [it means]: 'It is not sold and the money is [to be considered] a gift'; just as is the case of one who betroths his sister; for it was stated: If one betroths his sister, Rab said the [betrothal] money is to be returned, and Samuel holds that the money is regarded as a gift!<span class="x" onmousemove="('comment',' Everyone knows that he cannot betroth his sister, hence his form of betrothal was a humorous manner of giving her a gift. V. Kid. 46b.');">²</span>

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