The Torah forbids partaking of a vein in animals called the "Gid Ha'nashe," which is commonly identified as the sciatic nerve. The Zohar comments that this vein is forbidden not only for consumption, but also for any type of Hana'a (benefit). As such, it would be forbidden to sell the Gid Ha'nashe to a gentile. Butchers generally receive more profit by selling a whole piece of meat, and it is therefore profitable to sell that entire area of the animal's leg whole to a gentile. According to the Zohar, however, this would be forbidden because one may not derive any benefit whatsoever from this vein.
The Bet Yosef (commentary to the Tur by Maran, author of the Shulhan Aruch), however, based on the Gemara, writes that the prohibition of Gid Ha'nashe applies only to eating. According to the Talmud, it is permissible to derive other forms of benefit from the Gid Ha'nashe. It thus emerges that the Zohar and the Talmud disagree regarding the issue of deriving benefit from the Gid Ha'nashe that does not involve consumption.
Interestingly enough, the Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), who normally follows the rulings of the Zohar, writes that it is permissible to sell the Gid Ha'nashe. It would seem that if the Ben Ish Hai rules against the Zohar in this instance, then we can rest assured that normative Halacha follows the Gemara's ruling, that one may derive benefit from the Gid Ha'nashe. Indeed, Hacham Ovadia Yosef rules leniently in this regard, and it is thus permissible for a butcher to sell the sciatic nerve to a gentile.
A separate question arises concerning the use of this vein as stitching for the Torah scroll. Halacha requires that the parchments of the Torah scroll be stitched together with thread made from the veins of a kosher animal. According to some authorities, this means that the vein itself must be permissible for consumption, and therefore the Gid Ha'nashe is disqualified as stitching for the Sefer Torah. Others, however, interpret this Halacha as referring only to the species of the animal from which the vein was taken. So long as the animal is a kosher animal, one may use as stitching even a vein that the Torah forbids for consumption.
Halacha follows the stringent position, which forbids using a Gid Ha'nashe vein as stitching for the Torah scroll. Even though one may derive benefit from this vein, it may nevertheless not be used as stitching for a Torah since it is forbidden for consumption.
Summary: The Torah forbids partaking of the Gid Ha'nashe (sciatic nerve), but it is permissible to sell it to a gentile. This vein may not be used as stitching for a Torah scroll.
