There are different juristic views concerning one who intended to offer a sacrifice but could not do so during the days of sacrifice. The Hanafi scholars are of the opinion that he has to give the animal itself (i.e., alive) or its value in money in charity.
The Shafi`i and Hanbali scholars and some Maliki jurists however hold the view that there are different rulings for one who intended to sacrifice an animal as an act of sunnah and one who has made it obligatory by vowing to offer it. If the former could not sacrifice the animal in the specified time, he would miss the udhiyah of that year, and if he slaughtered it, he would not be rewarded for it as an udhiyah but, rather, as an act of charity for what he would give to the poor of its meat.
As for the one who has made the udhiyah obligatory for himself but could not offer it in the days of sacrifice, he has to make up for it, for obligating himself to offer it, he would not be exempted from this by the elapse of its due time before sacrificing it.
The Kuwaiti Encyclopedia of Fiqh states:
According to the Hanafi school, the point of offering the udhiyah as an act of getting close to Almighty Allah is to be achieved through slaughtering it in the specified time for that [during the period from after the `Eid Prayer on the Day of `Eid Al-Adha until the last of the Days of Tashreeq (13 Dhul-Hijjah)]. So, it is not to be made up for by means of slaughtering after the elapse of its time. If one could not offer one’s udhiyah in the specified time, one is to make up for this by giving in charity the udhiyah alive or its value in money, or a value of an animal sacrificeable as an udhiyah.
The point of getting Almighty Allah’s pleasure by sacrificing something of one’s property is mainly achieved by giving that thing in charity, but this is different in the case of udhiyah, where the sacrifice is to be achieved by slaughtering it in a specified time.
Note that when one slaughters the udhiyah, it is allowed for one and one’s family to eat and feed others, including rich people, from its meat. But when one misses the time for offering the udhiyah and, consequently, has to give it alive in charity, one is not allowed to slaughter it to make up for missing it. Should one slaughter it then, one would not be allowed to eat or feed rich people from it; one would be required then to give its slaughtered parts wholly in charity. If its value after being slaughtered was less than its value while alive, one is to pay the difference in value. Likewise, if one ate or fed a rich person or wasted a piece from it, one would pay the value of that piece.
If one vowed to offer an udhiyah but did not do so until its specified time had elapsed, and one came on the verge of death, one is to bequeath a value of an udhiyah in one’s will to be paid on one’s behalf. In this way, one can make up for missing the udhiyah that one had obligated oneself to offer.
As for the Shafi`i and Hanbali schools, they believe that if one intended to offer an udhiyah as an act of sunnah but the time of sacrifice elapsed before one could do so, one would not be obliged to make up for this; one would then merely miss the udhiyah of that year. If one slaughtered an animal to make up for this, one would not be rewarded for it as an udhiyah; however, one’s reward in that case would be of an ordinary act of giving. But if one vowed to offer an udhiyah and did not offer it until the time of sacrifice was over, one has to make up for it by slaughtering it. One is not to be exempted from this, for one has obliged himself to offer it by vowing. If it happened that the animal specified for the purpose of udhiyah was lost or stolen without one’s having been negligent in that regard, one would not be required to offer another animal as a sacrifice. If one later found the animal, one would sacrifice it to make up for the vow. But mind that, on sacrificing the animal in that case, one is not allowed to eat or feed one’s family or rich people of its meat.