Can I start fasting based on the moon sighting of another country?
You must follow the moon sighting of the country you are in.
What are the conditions for a valid Udhiyah?
Praise be to Allah, and peace and blessings be upon our master, the Messenger of Allah.
First: The age of the animal must meet the Sharia requirements. These requirements vary depending on the type of sacrifice:
Camels: Must have completed five years and entered their sixth.
Cows: Must have completed two years and entered their third.
Goats: Must have completed two years and entered their third. As for Sheep, they must have completed one year and entered their second.
Some scholars have permitted goats that have completed one year and entered their second.
The Hanafi school, along with an opinion in the Maliki school, permits sacrificing sheep that are at least six months old, provided they are healthy and physically substantial. According to the Shafi’i school, it is permissible if the sheep sheds its front teeth (ajdha') before reaching one year [Al-Iqna’, by Al-Shirbini (Vol.2/P.588)].
Second: Soundness and freedom from defects. The animal must be free from any defect that causes a decrease in its meat or market value. This is based on the hadith narrated by Al-Bara' bin 'Azib, that the Prophet (peace be upon him) said:
"Four [defects] are not permissible in sacrifices: A one-eyed animal whose blindness is evident, a sick animal whose illness is evident, a lame animal whose lameness is evident, and an emaciated animal that has no marrow in its bones." [Reported by Abu Dawood and Al-Tirmidhi, who graded it as authentic].
These defects are detailed as follows:
Evident Lameness: It is not permissible to sacrifice a lame animal if the lameness is severe enough to prevent it from walking to the pasture or seeking food, as this leads to a decrease in its meat. However, slight lameness that does not hinder its grazing is overlooked.
Evident Blindness (One-eyed): It is not permissible to sacrifice a sheep, cow, or camel that has a white film over its eye blocking light, or one that has lost an eye entirely. Weak vision that does not affect its ability to eat does not prevent the sacrifice from being valid.
Evident Illness: An animal with a clear sickness that prevents it from eating or moving is not valid. This includes severe mange (Jarab) that spoils the meat.
Extreme Emaciation: An animal so thin that there is no marrow left in its bones is invalid. The standard for emaciation that invalidates the sacrifice is that which spoils the quality of the meat to the point that people would find it undesirable even in times of plenty.
Additional Considerations:
These are the defects mentioned in the Prophetic tradition, and any defect that causes emaciation or reduces the meat or value is compared to them by analogy. This includes animals that are mentally unstable (diseased), those with mange, or those with a missing ear. In contrast, a slit or pierced ear does not affect the validity of the sacrifice. And Allah the Almighty knows best.
What is the ruling on offering an Udhiyah on behalf of another with their permission?
Praise be to Allah, and peace and blessings be upon our master, the Messenger of Allah.
It is permissible for a person to offer a sacrifice (Udhiyah) on behalf of someone else with their permission, even if the person performing it has not offered a sacrifice for themselves. It is stated in Sharh Manhaj al-Tullab (Vol.5/P.261) by Zakariya al-Ansari: "No one may offer a sacrifice on behalf of another without their permission... as opposed to when permission has been granted." 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.
