“It seems so, but there are some exceptions,” answered Rabbi Dayan. “The Bi’ur Halacha(s.v. afilu) writes that since the reason the esrog itself must be returned is because the owner needs it for the remainder of Sukkos, if the owner has another esrog with which to fulfill the mitzvah, the recipient can return the value when it’s no longer intact.”
“Then how about returning a different esrog?” asked Lev. “Would that be good enough?”
“Some authorities do not consider even this sufficient,” replied Rabbi Dayan. “However, the Bi’ur Halacha maintains that if you return an esrog that is kosher and of equivalent quality, you fulfill your stipulation. He also takes into consideration the more lenient position of some authorities; namely, that even regarding an esrog one can fulfill the stipulation by returning its value when the esrog is not intact. Further, when the item was ruined through oness, some maintain the recipient doesn’t have to pay. [See C.M. 241:8.] Thus, even the Shulchan Aruch might agree that you fulfill the stipulation when returning a comparable esrog.”