A Muslim should strive to use very pure things that are free from defects when offering it for the Sake of Allah. When offering a sacrificial animal (udhiyah), a Muslim should use animals that are free from defects. If the animal develops some defect after it is bought and allocated for the purpose of sacrifice, then it is to be used for the same purpose and there is no sin in that. However, one is not permitted to buy a defective animal for the purpose of sacrificing it. However, if the animal is created without a tail, for example, then it is permissible to use it.
The Standing Committee for Fatwa in the Kingdom of Saudi Arabia addresses this point:
A sheep whose fatty tail has been cut off is not fit to be offered in sacrifice because of the report narrated by `Ali ibn Abi Talib (may Allah be pleased with him), who said: “The Messenger of Allah (peace and blessings be upon him) commanded us to check the eyes and the ears, and not to sacrifice any sheep that was one-eyed or whose ears had been cut and some of it left hanging, or any whose ears had been cut from the back, or any whose ears had been torn or split.
This applies if it has been cut, but if the lamb was born without a tail in the first place, then it comes under the ruling of one that is created without horns and one that has a small tail. The ruling is that these animals are good enough for sacrifice.
But what should happen if, after buying a good animal for sacrifice, some defect happens to it? The fatwa of the late Sheikh Muhammad ibn `Uthaymeen, a prominent Saudi Muslim scholar (may Allah shower mercy on his soul), addresses this point:
The author of Zaad Al-Mustaqni` said: “If it (the sacrifice) gets injured (develops a flaw), it may still be slaughtered and this be accepted…”When a man buys a sheep to sacrifice it, then it breaks its leg and is unable to walk as well as the rest of the flock, it may be slaughtered and it will be accepted from him because when it was selected it became a trust, like something that was entrusted to him. Because it is something entrusted and the injury was not the result of his action or his negligence, he is not obliged to offer any guarantee, and it will yield reward in sha’ Allah.