All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
First of all, it should be mentioned that Shaykh Sayyid Saabiq said in his book Fiqh-us-Sunnah that the rennet of dead animals (Maytah) is permissible, not what you said, 'of unlawful animals', because the unlawful animals include dogs and pigs. That is not what he meant; rather, he meant the rennet of the animals whose meat we are allowed to eat, like cows and sheep (but which were Maytah; i.e. not slaughtered according to the Sharia).
The Fiqh Encyclopedia defines rennet (Arabic: Infahah or Minfahah) as follows: “It is a yellowish-white substance in a skin pouch that is extracted from the stomachs of suckling kids or lambs. When a little of this substance is added to milk, it curdles and becomes cheese. In some Arabic-speaking countries, people call this rennet Majbanah (cheese-maker), and the stomach (from which the rennet is taken) is called Karish if the animal grazes on grass.”
Scholars of Fiqh differed in opinion about the ruling on rennet; some of them absolutely forbade it; some of them forbade the liquid rennet but not the solid rennet; and some of them totally permitted it. This last view is the view chosen by Sayyid Saabiq in his book Fiqh-us-Sunnah.
The Fiqh Encyclopedia reads:
“Scholars of Fiqh held three different opinions regarding the ruling on the rennet of a dead animal whose meat we are allowed to eat, whether it is impure or pure, as it may be used in making cheese:
The First Opinion: the view of the majority of scholars from the Maaliki and Shaafi'i schools and the predominant view of the Hanbali school is that rennet is impure and the cheese that is made with it is impure and it is not permissible to eat it. The fact that the animal is unlawful to eat means that all its parts are unlawful to eat as well, including the rennet.
The Second Opinion: the view of Abu Yoosuf and Muhammad, who were the two disciples of Abu Haneefah, is that if the rennet is liquid, then it is impure because its vessel is impure, but if it is solid, then it is washed from the outside and eaten.
The Third Opinion: the view of Abu Haneefah and Ahmad in one of his narrations that is given preponderance by Ibn Taymiyyah is that the rennet is pure, because when the Companions entered Persia, they ate the cheese made there, which was made from rennet taken from kid goats, while their animals were dead animals (Maytah).”
There is no doubt that the view that rennet is pure is a considerable view, but it is better to act according to the view of the majority of scholars in order to be on the safe side.
As regards why we do not apply this ruling to milk and hormones and other derivatives from dead animals, then the difference of opinion about the milk of dead animals is like the difference of opinion about the rennet. Shaykh Ibn Taymiyyah said, “Regarding the milk of dead animals and their rennet, there are two famous opinions of the scholars: one view is that they are pure, as is the view of Abu Haneefah and others, and it is also one of the two narrations of Imaam Ahmad; the second view is that they are impure, like the view of Ash-Shaafi’i and the other narration from Ahmad. It is based on this difference of opinion that they based their different views about the cheese of the Magi…”
He said:
“The apparent view is that the milk and rennet of dead animals are pure, because when the Companions conquered the lands of Iraq, they ate from the cheese of the Magi; this was something acceptable among them. What was reported, that some of them disliked it, requires scrutiny, as it was reported from some of the scholars of Hijaaz (Makkah and Madeenah), and it requires scrutiny. The people of Iraq were more knowledgeable about this as the Magi lived in their lands and not in the lands of Hijaaz. The evidence for that is that Salmaan Al-Faarisi was the deputy of ‘Umar ibn Al-Khattab over Madaa'in (capital of Persia at the time), and he was calling the Persians to Islam. It was confirmed that Salmaan Al-Faarisi was asked about animal fat, cheese, and fur, and he replied, ‘The lawful is what Allah has made lawful in His Book, and the unlawful is what Allah has made unlawful in His Book. Whatever He has kept silent about is pardoned.’ [Abu Daawood narrated this hadeeth as Marfoo‘, i.e. directly attributed to the Prophet, sallallaahu ‘alayhi wa sallam]
It is known that the question was not about the cheese of the Muslims and the People of the Book, as this is a known matter. Rather, the question was about the cheese of the Magi; this is evidence that Salmaan used to issue the fatwa that it was lawful.”
There is no doubt that this issue is subject to Ijtihaad (independent reasoning), and there is no harm on a person to imitate either of the two opinions.
Even though Shaykh Ibn Taymiyyah held that rennet is pure, he said, “Each of the two views relies on the statements of the Companions, and the scholars who held the first opinion reported that the Companions ate from the cheese of the Magi, and the scholars who held the second opinion reported that they ate what they believed to be the cheese of the Christians. So, this is an issue of Ijtihaad, and a person may follow either of the two views…”
Regarding the other extracts, if they never had life in them (i.e. could be described as alive or dead), then they are subject to the same difference of opinion as milk and rennet. The scholars who held that milk and rennet are permissible argued that they (milk and rennet) did not die because they were not described as alive in the first place (hence they are not considered Maytah). Ibn Taymiyyah said, “Also, the milk and rennet did not die; rather, the scholars who held that they are impure chose this opinion because they (milk and rennet) are contained in an impure vessel...” If the extracts could be described as alive or dead, then they are unlawful if they come from a dead animal (Maytah), just like the meat of the dead animal.
Allah knows best.