No Alternate for Qurbani
Some people think that instead of offering a Qurbani they should give its amount to some poor people as charity. This attitude is totally wrong. Actually, there are different forms of worship obligatory on Muslims. Each one of them has its own importance and none of them can stand for the other. It is not permissible for a Muslim to perform salah instead of fasting in Ramadan, nor is it permissible for him to give some charity instead of observing the obligatory Salah. Similarly, Qurbani is an independent form of worship and this obligation cannot be discharged by spending money in charity.
However, if somebody, out of his ignorance or negligence, could not offer Qurbani on the three prescribed days (10th, 1lth and 12th Zulhijjah) then, in that case only, he can give the price of a Qurbani as sadaqah to those entitled to receive Zakah. But during the days of Qurbani no Sadaqah can discharge the obligation.
The Animals of Qurbani
The following animals can be slaughtered to offer a Qurbani:
1. Goat, either male or female, of at least one year of age.
2. Sheep, either male or female, of at least six months of age.
3. Cow, ox buffalo of at least two years of age.
4. Camel, male or female, of at least five years of age.
One head of goat or sheep is enough only for one person's Qurbani. But as for all other animals like cow, buffalo or camel, one head of each is equal to seven offerings thus allowing seven persons to offer Qurbani jointly in one such animal.
If the seller of animal claims that the animal is of the recognized age and there is no apparent evidence to the contrary; one can trust his statement and the sacrifice of such an animal is lawful.