(a) TOSFOS (Kidushin 63a) uses this case of a woman prohibiting her earnings
to her husband by being Makdish her hands to explain the case of one who
sells a tree for its fruit ("Dekel l'Peirosav"). The Gemara says that even
though the fruit that has not yet grown on the tree is a "Davar she'Lo Ba
l'Olam," one can sell that fruit by selling the *tree* for the sake of its
fruit. Although the future produce of a tree is a "Davar she'Lo Ba l'Olam,"
if one emphasizes that the *tree* is being sold (with regard to its
fruit-producing quality) and not the future produce itself, then the sale no
longer involves a "Davar she'Lo Ba l'Olam."
Tosfos in a number of places (see Gitin 66a) seems to explain that the sale
is actually taking place with the tree itself; it is as if the buyer has
become a partner in the ownership of the tree, and hence when the fruits
later grow he does not receive them from the seller, but rather they are
already his by virtue of his ownership of the tree.
This explanation, though, is problematic. How can Tosfos compare the case of
"Dekel l'Peirosav" to the case of the woman who is Makdish her hands in
order to prohibit her earnings to her husband? First, unlike the fruit of a
tree, one's earnings are not direct outgrowths of the hands. Second, unlike
the tree, the hands themselves cannot be sold, nor can they actually become
Hekdesh!
(b) Due to these questions, the RASHBA (Kesuvos 58b, Gitin 42b, and numerous
places in his Teshuvos) offers a different explanation to why the sale of
"Dekel l'Peirosav" circumvents the problem of "Davar she'Lo Ba l'Olam." He
explains that the sale does not transfer partial ownership of the tree to
the buyer, because -- although that explanation suffices for the case of
"Dekel l'Peirosav" -- it does not explain other cases mentioned in the
Gemara, such as the purchase of a slave with regard to the buyer receiving
the money of the Kenas in the eventuality that someone kills him and is
Chayav to pay the Kenas to the slave's owner. The potential Kenas is
certainly not an element that exists at present; it is a Chiyuv that occurs
if such a situation happens to arise (see CHIDUSHEI RABEINU CHAIM HA'LEVY,
Hilchos Mechirah).
Rather, the Rashba explains that the sale of "Dekel l'Peirosav" *is* a sale
of the produce itself, similar to the sale of "Peiros Dekel." How, though,
does it work, if the fruit has not yet come into the world? The Rashba
explains that by making the formal Kinyan on the tree, which does exist at
present, the tree serves as the vehicle with which to complete the sale of
the produce and thereby circumvents the problem of "Davar she'Lo Ba l'Olam."
That is, the *sale* is actually being made on the non-existent fruits, while
the *Kinyan* to effect that sale is being made on the existent tree (this is
conceptually similar to a Kinyan Sudar ("Chalipin"), whereby the Kinyan is
made on a handkerchief while the sale takes effect on an object that is
elsewhere). This, he reasons, can be applied to the case of the slave, as
well as to our case of the woman's hands, even though the earnings are not a
product of the hands nor can the hands be sold or made into Hekdesh
outright. The woman is making her hands the vehicle by which the Neder takes
effect on her earnings.