If it is valid to isolate it, it prevents [the sale], and if it is not valid to isolate it—like a decorated sword sold for the genus of its decoration—then that which does not prevent [the sale] does not prevent it, even if its isolation is valid, like the slave's property.

Section: If one sells a genus in which usury applies for its own genus, and each contains something of a different genus that is not intended, this divides into categories. The first is that the unintended [component] is minor, not affecting volume or weight, such as salt in things it is processed with, or grains of barley in wheat; it does not prevent [the sale] because it is minor and does not interfere with the equality [required]. Likewise, if it is found in one of them but not the other, it does not prevent it for this reason. If one sells that for the genus of the unintended component that is with it—such as selling bread for salt—it is permissible, because its presence is as its absence. The second is that the unintended component is major, yet it is for the benefit of the intended one, like water in date vinegar, raisins, and date molasses. It is permissible to sell a portion of it for its like, and its mixture is treated as its moisture because it is for its benefit, so it does not prevent selling it for what is equivalent to it, like fresh dates for fresh dates. It is not permissible to sell it for what does not contain such a mixture, like selling grape vinegar for raisin vinegar, because this leads to disparity (tafadul), so it runs the course of selling dried dates for fresh dates. Al-Shafi'i prohibited all of this, except for selling sesame oil for sesame oil, because the water does not appear in the sesame oil. The third is that the unintended component is major and is not for its benefit, like milk mixed with water, and currencies adulterated with other materials. It is not permissible to sell some of it for others because its mixture is not for its benefit, and it interferes with the equality that is intended. If one sells it for the genus of the unintended component, such as selling a debased dinar [made of gold] mixed with silver for dirhams, the permissibility is possible because he is selling it for a genus not intended in it, which resembles selling milk for a sheep that has milk in it; the prohibition is also possible, based on the other opinion regarding the original issue. If one sells a debased dinar for its like, and the adulteration in both is different or its amount is unknown, it is not permissible because it interferes with the required equality. If one knows of the equality in the gold and the bullion that they contain, it is derived from the two opinions; the most appropriate of which is permissibility, because they