The heart is one category, and the brain/marrow is one category. It is permissible to sell every category for another category with a surplus (tafadul). Al-Qadi said: It is not permissible to sell meat for fat. Malik disliked that, unless they are equal. The apparent position of the school (Madhab) is the permissibility of selling them both as equals or with a surplus, and this is the opinion of Abu Hanifa and al-Shafi'i; because they are two distinct genera, so a surplus is permissible between them like gold for silver. If one prohibits it due to meat not being free from fat, it would not be valid, because the fat does not appear, and if there is something of it, it is not intended, so it does not prevent the sale. If one were to prohibit it for that reason, then selling meat for meat would not be permissible, due to each of them containing what is not of its genus. Then, this is not valid according to al-Qadi, because the fat that exists with the meat is considered meat according to him, so it is not conceivable that meat contains fat. Al-Qadi mentioned that the white meat which is on the exterior of the red meat [is, along with the red,] one genus, and that the tail fat (alya) and the fat (shahm) are two genera. The apparent statement of al-Khiraqi is contrary to this, for his saying: "Indeed, meat is not free from fat." If this were not considered fat, then meat would not be mixed with fat. Therefore, according to this, every white [substance] in the animal that melts upon heating and becomes grease is one genus. This is more correct, due to the saying of the Almighty: "We have forbidden them their fats except what their backs carry" (Quran 6:146). He excepted what the backs carry from the fat, and because it resembles fat in its melting, its color, and its intended use, it is therefore fat, like that which is in the abdomen.

Section: Regarding milk, there are two narrations. One of them is that it is one genus, for what we have mentioned regarding meat. The second is that it is multiple genera due to the difference in its origins, like meat. This is the school of al-Shafi'i, and Malik also said this; because livestock as a whole are one genus. Ibn Aqil said: The milk of domesticated cattle and wild cattle