This excerpt is taken from Sheikh Farooq Rafi Sahib's book Qurbani, Aqiqa, and Ashra Zil-Hijjah.
If a defect occurs in the sacrificial animal after purchase, then sacrificing such an animal is not permissible and it must be replaced, because being free from legal defects is a condition for sacrifice. It is not necessary for the animal to be free from defects at the time of purchase, but it is required to be free from these defects at the time of slaughter, and slaughtering an animal afflicted with the mentioned defects is prohibited.
Also, the narration from which the implication is drawn that if the sacrificial animal becomes defective after purchase, its sacrifice is permissible, is weak.
Sayyiduna Abu Saeed Khudri (may Allah be pleased with him) narrates:
ابتعنا كبشا نضحي به، فأصاب الذئب من إليتيه وأذنه، فسألنا النبى صلى الله عليه وسلم فأمرنا أن نضحي به
“We bought a ram for sacrifice, and both its thighs and both ears were eaten by a wolf. So we asked the Prophet (peace be upon him) about its sacrifice, and he instructed us to slaughter that very ram.”
Reference: Very weak: Sunan Ibn Majah, Chapters on Sacrifices, Chapter on One Who Buys a Sound Sacrifice and Something Happens to It: 3146. Musnad Ahmad, 78/3. Jabir bin Yazid bin Harith Al-Juhani is abandoned, and Muhammad bin Qariyah bin Ka'b Al-Ansari is unknown, and his hearing from Abu Saeed Khudri is also not established.
