Is it permissible to purchase the Udhiyah on installments?
It is permissible to purchase the sacrificial animal (uḍḥiyyah) on installments or by borrowing its price. However, it is not recommended for the poor to do so, because they are not required to offer a sacrifice, and Allah does not burden a soul beyond its capacity. If doing so would lead to negligence in providing for one's dependents (nafaqah wājibah), then giving precedence to obligatory maintenance over borrowing to buy the sacrifice is appropriate. This is based on the saying of the Prophet (peace and blessings be upon him): "It is sufficient sin for a person to withhold food from those whom he is responsible to support."
Furthermore, the sacrifice is valid from one who has a debt, but it is preferable (awlā) to repay the debt first, especially if the debt is due immediately (ḥāll). And Allah Almighty knows best.
Is it permissible to include others in the reward of one`s sacrifice?
In the Name of Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
It is permissible for the person offering an Udhiyah (sacrificial offering) to include others in the spiritual reward of the sacrifice. However, it is not permissible for multiple people to jointly share the purchase price of a single sheep or goat, unless one person gifts the money to the other.
It is stated in Mughni al-Muhtaj (6/137) by Al-Khatib al-Shirbini:
"If a person includes someone else in the reward of his sacrifice and slaughters it on his own behalf, it is permissible."
It should be carefully noted that including someone else in the reward of one's sacrifice is completely distinct from offering a sacrifice on behalf of someone else. And Allah the Almighty Knows Best.
What is the ruling on offering a sheep as a sacrifice (Udhiyah) if its fat-tail is sound, except that when it was young, the tip of its fat-tail was cut so that it would grow larger? And what is the ruling in case of doubt regarding the amount that was cut?
In the Name of Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
Cutting a minor, insignificant portion from the tip of an animal's fat-tail (al-alyah) to encourage it to grow larger is not considered a defect, and it does not prevent the animal from being valid for sacrifice (Udhiyah).
It is stated in Tuhfat al-Muhtaj (Vol.9/P.352): "There is some scholarly deliberation regarding the common practice of cutting the tip of the fat-tail so that it grows larger. It could potentially be likened to a partial cut of the ear—supported by the jurists' general rule: 'even if it is a small amount.' On the other hand, if it is an exceptionally minor cut, it might have no effect on validity. This is explicitly clarified by the juristic exception to the general rule, which states that cutting a tiny piece from a large limb causes no harm. This latter view is more well-founded.
Furthermore, I found that some scholars investigated this matter and concluded: 'It should not affect validity if a custom-sanctioned portion of its fat-tail is removed during its youth to make it grow larger and look better, just as castrating a male animal causes no harm.' However, applying this unconditionally contradicts the established texts of the jurists, as understood from what I have laid out; thus, the restriction I specified is what must be relied upon."
Similarly, it is mentioned in Nihayat al-Muhtaj (8/135): "If a small piece is cut from the fat-tail to help it grow larger, the most well-founded view is that the sacrifice remains valid, as was given in a formal legal verdict (Fatwa) by my father [Shihab al-Din al-Ramli], may Allah be pleased with him. This is proven by the jurists' maxim: 'The loss of a tiny piece from a large limb causes no harm.'"
In cases where there is doubt as to whether the portion cut was large or small, the animal is still deemed valid for sacrifice. It is noted in Hashiyat al-Shubramallisi ‘ala Nihayat al-Muhtaj (Vol.8/P.135):
"This matter requires careful consideration, but the closer and more correct view is that it is valid. This is because soundness is the default state for the animal from which the piece was cut, and it aligns with what usually occurs—namely, that the part removed to help the fat-tail grow larger is naturally very small." And Allah the Almighty Knows Best.
What is the ruling on congregational prayer performed outside the mosque, and is it a condition that it be performed in a mosque in order to attain the full reward?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
It is not a condition for attaining the reward of congregational prayer that it be performed in a mosque; however, a Muslim should still be keen to perform his prayers in the mosque, so as not to miss out on immense reward. This is due to the virtue of the mosque and of spending time there (i‘tikāf), the typically larger number of worshippers present, and the additional reward gained through walking to and from the mosque. On the authority of Abū Hurayrah (raḍiyallāhu ‘anh), the Messenger of Allah ﷺ said: "Whoever purifies himself in his house, then walks to one of the houses of Allah to fulfill one of the obligations Allah has prescribed, one of his steps will erase a sin, and the other will raise him a degree [in status]." (Related by Muslim)
Whoever misses congregational prayer at the mosque should still strive to pray in congregation at his workplace, or together with the members of his household. And Allah, the Exalted, knows best.