The author of Zaad al-Mustaqni’ said: “If it (the sacrifice) gets injured (develops a flaw), it may still be slaughtered and this will do…” 

Shaykh Ibn ‘Uthaymeen said: an example of that is when a man buys a sheep to sacrifice it, then it breaks its leg and is unable to walk and keep up with the rest of the flock, after he has selected it for sacrifice. In this case it may be slaughtered and this will do, 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, so he is not obliged to offer any guarantee, and it will do. 

See al-Sharh al-Mumti’, part 7, p. 515