Is it permissible for one who sacrifices on behalf of another, with the latter's permission, to eat from the sacrifice?
It is permissible for someone who sacrificed on behalf of another with their permission to eat from it with their permission, and they stand in their place (act as their representative) in distributing it.
It is stated in Nihayat al-Muhtaj ila Sharh al-Minhaj (Vol.8/P.141): "And he—meaning the one sacrificing on his own behalf, provided he has not apostatized—has the right to eat from a voluntary sacrifice and his sacrificial gift; rather, it is recommended. As for an obligatory sacrifice, it is forbidden for him to eat from it, whether it was designated as such initially or as a liability in his dhimmah. And excluded by what has preceded is if he sacrifices on behalf of another, or if he apostatized, then it is not permissible for him to eat from it, just as it is absolutely impermissible to feed a disbeliever from it."(The text I found indicates the impermissibility of eating) ???
And it is stated in Hashiyat al-Jamal ‘ala Sharh al-Manhaj (Vol.5/P.262): "If a person offers a sacrifice on behalf of a living person with their permission, does he act as their representative in distributing it—on the grounds that permission to sacrifice is permission to distribute it—or does it depend on their explicit permission? This requires consideration, but the first view is not far-fetched." And Allah the Almighty knows best.
What is the ruling of Islamic Law on the prayer of zawal?
All praise is due to Allah, and peace and blessings be upon our master the Messenger of Allah ﷺ.
It is recommended (sunnah) to pray four rakʿāt — either with a single tasleem or as two separate sets of two rakʿāt — immediately following the sun's decline from its zenith (zawāl). This prayer is distinct from the regular Sunnah prayer of Ẓuhr (sunnat al-ẓuhr al-rātibah), as explicitly stated by the Shāfiʿī jurists.
It is mentioned in Nihāyat al-Muḥtāj: "The prayer of zawāl is offered after the sun's decline — so were one to perform it before that, it would not count. It consists of two or four rakʿāt and is distinct from the Sunnah of Ẓuhr, as is evident from the fact that it is mentioned separately after the regular Sunnah prayers, and it becomes a make-up prayer (qaḍāʾ) if a long period of time passes by customary reckoning... Al-ʿAlqamī stated: 'Scholars refer to this as the Sunnah of Zawāl, and it is distinct from the four rakʿāt that constitute the Sunnah of Ẓuhr.' Our shaykh said: Al-Ḥāfiẓ al-ʿIrāqī stated that among those who explicitly affirmed its recommendation was al-Ghazālī in al-Iḥyāʾ, in the chapter on devotional litanies, noting that there is no tasleem between them — meaning there is no break between each pair of rakʿāt."
The time of the sun's decline (zawāl) marks the very beginning of the time for the Ẓuhr prayer.
And Allah the Almighty knows best.
Is the one who gives up on marriage because he can`t afford it considered sinful?
Marriage is desirable, but one who can`t afford it isn`t considered sinful. However, he should supplicate to Allah, and seek His forgiveness.
What is the ruling of Islamic Law regarding one who purchases a sacrificial animal (uḍḥiyah) and it then develops a defect before slaughter?
Praise be to Allah, and peace and blessings upon our master, the Messenger of Allah.
If a defect that invalidates the sacrifice (uḍḥiyah) arises after its purchase — for example, if one purchased a sound, defect-free animal, and it then developed a limp, blindness in one eye, or a similar defect before slaughter — it does not fulfill the requirement of a valid uḍḥiyah, according to the Shāfi'ī school.
It is stated in Asnā al-Maṭālib fī Sharḥ Rawḍ al-Ṭālib (Vol.1/P.535): "Even if the limp develops [in the animal] while the knife is upon it, it still does not fulfill the requirement, because it is lame at the moment of slaughter — this is analogous to a case where a sheep's leg breaks and one hastens to slaughter it [in that condition]."
The Ḥanbalī school, however, held that if the one offering the sacrifice purchased the animal while it was sound and defect-free, and a defect then befell it afterward, the sacrifice remains valid and there is no obligation to replace it.
It is stated in Masā'il al-Imām Aḥmad, one of the Ḥanbalī reference works (Vol.8/P.4021): "I said: If a person purchases the sacrificial animal while it is sound, and it is then afflicted with illness, blindness in one eye, or a broken limb [before slaughter]? He [Imam Aḥmad] said: It is said that it still fulfills the requirement. Isḥāq said likewise, because he purchased it while sound, and the defect befell it only afterward, so it remains sufficient on his behalf." [End of quote]
Accordingly, a sheep afflicted with a defect that invalidates the sacrifice does not fulfill the requirement of a valid uḍḥiyah — whether the defect arose after purchase or during the slaughter itself — according to the Shāfi'ī school. However, there is no objection to following the Ḥanbalī position on this matter [as a valid alternative]. And Allah, the Most High, knows best.