The general ban on taking money for a dog admits one exception: dogs it is lawful to keep may be sold for their lawful use. As for the prohibition on the cat’s price, the ḥadīth establishing it is authentic, yet the majority of jurists deem its sale permissible, maintaining that the cat is intrinsically pure and its benefits are licit. Their disagreement over dogs, therefore, stems from the clash of evidences.

A related debate concerns the sale of impure oil and similar substances. Although the scholars are unanimous that eating such oil is forbidden, they differ over its sale. Mālik declared, “The sale of impure oil is invalid,” and al-Shāfiʿī agreed. Abū Ḥanīfah, however, held that its sale is permissible provided the impurity is disclosed, a view also reported from Ibn Wahb among Mālik’s followers.

Those who prohibit the sale cite the aforementioned ḥadīth of Jābir, who heard the Messenger of Allah—peace and blessings be upon him—on the day of the Conquest say: “Allah and His Messenger have forbidden wine, carrion, and swine.”

Those who allow the sale argue that when a single item provides several benefits, the presence of one prohibited benefit does not automatically render the others unlawful—especially when the lawful benefit is needed as much as the unlawful one. On this principle, wine, carrion, and swine are singled out for total prohibition, while other creatures whose flesh may not be eaten remain permissible to sell so long as the sale is for a benefit other than consumption. They narrate from ʿAlī, Ibn ʿAbbās, and Ibn ʿUmar that they allowed the sale of impure oil to be used for lighting.

Within the Mālikī school, it is permitted to burn such oil in lamps or to make soap from it, while its sale remains forbidden; al-Shāfiʿī likewise allows those uses yet disallows taking payment. All of this, however, is considered weak. Some even relate another report in the school that forbids lighting with it altogether, a stance more consistent with the original principle—the prohibition of selling it.

The school also records two opinions on whether washing or boiling the oil affects the impurity itself and removes it. One opinion allows this; the other forbids it. The disagreement turns on whether impurity that mixes with the oil becomes inherent (najāsat ʿayn) or is merely contiguous (najāsat mujāwara). Those who view it as contiguous deem it purified by washing or boiling, whereas those who deem it inherent do not.

Another well-known issue in this chapter is the permissibility of selling expressed human milk. Mālik and al-Shāfiʿī allow the sale, while Abū Ḥanīfah forbids it. Those who permit argue that, since drinking it is lawful, its sale is likewise lawful, by analogy with the milk of livestock. Abū Ḥanīfah contends that its consumption is allowed only out of an infant’s necessity and that, in principle, it is forbidden—human flesh being forbidden—and that milk follows meat in this regard. They reason: just as the human being is an animal whose flesh may not be eaten, so his milk may not be sold; the same applies to pig’s milk and donkey’s milk.

Thus, in this chapter the root cause of disagreement is the collision of competing analogies. The subsidiary questions are many, but we have mentioned only the most celebrated issues in each section so that they may serve as guiding principles.