Arachin 19 - My Weight - To the Temple!
If one says, "I undertake to pay my weight to the Temple," then he must give it in the substance he specified, whether it be gold, silver, or even pitch - if in his place they sell pitch by weight. There was an incident when the mother of Yirmatya promised to pay the weight of her daughter, but did not specify the material. Since she was fabulously rich, they estimated that she had to pay in gold. The weight is taken at the time and hour of the vow.
If one promised the weight of his arm to the Temple, they measure the water displaced by his arm, and add donkey's flesh, bones, and sinews, until the water riches the same level. Rabbi Yose says that even though donkey meat is a good approximation, it will never be precise, therefore, they just estimate.
If he promised the worth (not weight) of his arm to the Temple, they estimate how much he is worth when working with one hand tied up; in this vows are more stringent than a symbolic value of hand - which is zero, since it is not a vital organ.
Art: Elisabeth Vigee-Lebrun - Madame Rousseau and her Daughter