Commentary for Kiddushin 91:9

Rav and Shmuel return to saying that the last clause refers to the first clause of the mishnah, where he says “Be betrothed to me with this one, be betrothed to me with this one.” If she puts the dates down it is obvious that she is not betrothed unless one is worth a perutah because she has not yet derived any benefit from them. But if she is eating them, and deriving immediate benefit, I might have thought that even if no date is worth a perutah, she has agreed to be taken in marriage. Therefore, the mishnah teaches that this is not so. A woman never agrees to be betrothed for less than a perutah.