http://revach.net/article.php?id=1055

Section: Halacha   Category: Tshuvos
How Much Do You Pay For Ruining Your Friend's Picture Perfect Esrog?

If someone damages his friend's Esrog and causes it to become Pasul, the Maharam Mintz holds that even if the damaged Esrog was very beautiful and expensive he only has to pay the cost of a simple non-Mehudar Esrog. The reason is because the Gemara says if you injure the Olah of your friend you are only obligated to pay for the cost of a lamb even though the Olah that you killed was a more expensive ox.

The Chacham Tzvi and the Mishneh l'Melech disagree however and they hold that you have to pay the price of a Mehudar Esrog. Even though regarding an Olah you only have to pay the price of a lamb and not of an ox that is because it is forbidden to derive benefit from an Olah it and thus it is not considered the possession of the Kohen. An Esrog however, is the possession of its owner and you therefore must pay its full value.