1) CIRCUMVENTING THE PROBLEM OF "DAVAR SHE'LO BA L'OLAM"
OPINIONS: The Gemara assumes that a person's future earnings are considered a "Davar she'Lo Ba l'Olam," and for that reason he cannot make a Neder to prohibit his earnings to someone else. The Gemara asks that if one's earnings are considered a "Davar she'Lo Ba l'Olam," why may a wife prohibit her earnings to her husband? The Gemara answers that the woman is able to prohibit her earnings to her husband by saying, "My hands should be consecrated (Hekdesh) to their Creator." This wording circumvents the problem of "Davar she'Lo Ba l'Olam" because her hands do exist at present.
(a) TOSFOS in Kidushin (63a) uses the Gemara's explanation of the case of a woman who prohibits her earnings to her husband (by consecrating her hands to their Creator) to clarify the case of one who sells a tree for its fruit ("Dekel l'Peirosav"). The Gemara says that although the fruit which has not yet grown on the tree is a "Davar she'Lo Ba l'Olam," one may 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, the sale no longer involves a "Davar she'Lo Ba l'Olam."
Tosfos in a number of places (see Gitin 66a) seems to understand that the sale actually takes place with the tree itself; it is as if the buyer has become a partner in the ownership of the tree. When the fruits grow later he does not receive them from the seller, but rather they are already his by virtue of his ownership of the tree.
Tosfos' comparison of the case of "Dekel l'Peirosav" to the case of the woman who consecrates her hands in order to prohibit her earnings to her husband is problematic. One's earnings are unlike the fruit a tree produces, because one's earnings are not direct outgrowths of the hands like the fruit is a direct outgrowth of the tree. Moreover, unlike the tree, the hands themselves cannot be sold or actually consecrated as Hekdesh.
(b) The RASHBA (Kesuvos 58b, Gitin 42b, and numerous places in his Teshuvos) offers a different explanation for why the sale of "Dekel l'Peirosav" circumvents the problem of "Davar she'Lo Ba l'Olam." He explains that the reason why there is no problem of "Davar she'Lo Ba l'Olam" is not that the sale transfers partial ownership of the tree to the buyer. Although that explanation suffices in the case of "Dekel l'Peirosav," it does not explain other cases mentioned in the Gemara, such as one who purchases a slave for the purpose of receiving the money of the Kenas in the eventuality that someone kills him and becomes obligated to pay the Kenas to the slave's owner. The potential Kenas is certainly not an element which exists at present. It is an obligation that is created if such a situation later arises. (See CHIDUSHEI RABEINU CHAIM HA'LEVI, 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 the sale take effect, if the fruit has not yet come into the world? The Rashba explains that when the buyer makes a formal Kinyan on the tree, which does exist at present, the tree serves as the vehicle to complete the sale of the produce. That is, the sale itself is being made on the non-existent fruits, while the Kinyan to effect that sale is being made on the existent tree. (This approach is conceptually similar to a Kinyan Sudar ("Chalipin"), whereby the Kinyan is made on a handkerchief while the sale takes effect on an object in another place.) He reasons that this approach applies to the case of the slave, as well as to the case of the woman's hands even though the earnings are not a product of the hands and the hands cannot be sold or made into Hekdesh outright. The woman makes her hands the vehicle through which the Neder takes effect on her earnings.