Can one Person use a cow or camel for Udhiyah

It is permissible for seven people to share a cow for Udhiyah. If it is permissible for seven to share a single cow, then it is even more permissible for fewer than seven to share it, with the additional share being voluntary, just as if one person were to slaughter a cow for Udhiyah, even though a sheep would suffice.

 
Imaam ash- Shaafi’ee (may Allaah have mercy on him) said: “If they are less than seven, it suffices them, and they are considered to be giving the extra portion as charity, just as a camel suffices for someone obligated to give a sheep, and they are considered to be giving the extra portion as charity instead of the sheep.” [Al-Umm, 2/244]

Al-Kasaanee said:  “There is no doubt about the permissibility of sacrificing a camel or a cow for fewer than seven people, whether two, three, four, five, or six share in a camel or a cow; because since seventh is permissible, then more is even more so. It makes no difference whether the shares are equal in quantity or different, such that one of them gets half while the other gets a third, and the other gets a sixth, provided that it does not fall below seventh.” [Badaa’ as-Sanaa’ 5/71].

Is It Required for a Man to be Just Between his Wives’ Families?

Question: Does the charity or gifts that a husband gives to his other wife’s family affect his fairness between his wives?

Shaykh Ibn Baaz (may Allaah have mercy on him) said: That’s fine, there is no harm in it, inshaa Allaah. It does not affect anything. The important thing is to treat them [i.e. his wives] equally and fairly in their own right. However, if he gives [a gift] to the family of one of his wives because of their poverty, or because of their kindness to him, or for other reasons, then this has nothing to do with treating his wives equally and fairly, and there is no harm in that. Yes.

 

Source: Fataawaa Noor alaa ad-Darb by Shaykh Abdul-Azeez ibn Abdullaah ibn Baaz; translated by Abdus-Samee Abdus-Salaam