Bava Batra 137B

אי מיפק לא נפיק ביה, למאי יהביה ניהליה?

if he does not fulfill his obligation with it, for what purpose did the owner give it to him?

אלא מיפק ⁠– דכולי עלמא לא פליגי, דנפיק ביה; מכרה או אכלה ⁠– באנו למחלוקת רבי ורבן שמעון בן גמליאל.

Rather, with regard to fulfilling his obligation, everyone agrees that he fulfills his obligation with it. It is only in a case where the first recipient sold it or consumed it, that we have arrived at the dispute between R' Yehuda HaNasi and Rabban Shimon ben Gamliel. According to R' Yehuda HaNasi, if he sold it, the second recipient can repossess the etrog from the purchaser, and if he consumed it he must pay the second recipient its value, whereas according to Rabban Shimon ben Gamliel, he owes nothing to the second recipient, as the etrog belongs to him.

אמר רבה בר רב הונא: האחין שקנו אתרוג בתפוסת הבית; נטלו אחד מהן ויצא בו ⁠– אם יכול לאוכלו, יצא; ואם לאו, לא יצא.

Rabba bar Rav Huna says: In a case of brothers who acquired an etrog with money from the jointly held property of the estate, and one of them took it on Sukkot and attempted to fulfill his obligation with it, the halakha depends on the limits of his ownership of the etrog: If he may eat it, i.e., his brothers allow him to do so, he has fulfilled his obligation to perform the mitzva, and if not, he has not fulfilled his obligation.

ודוקא דאיכא אתרוג לכל חד וחד, אבל פריש או רמון ⁠– לא.

And this is specifically in a case where there is an etrog for each and every one of the brothers; but if the other brothers get merely a quince or a pomegranate, he has not fulfilled his obligation even if he has the right to eat it.

אמר רבא: ״אתרוג זה נתון לך במתנה, על מנת שתחזירהו לי״, נטלו ויצא בו; החזירו ⁠– יצא, לא החזירו ⁠– לא יצא. קא משמע לן דמתנה על מנת להחזיר ⁠– שמה מתנה.

Rava says: If one person said to another: This etrog is given to you as a gift on the condition that you return it to me, and the recipient took it on Sukkot and attempted to fulfill his obligation with it, if he ultimately returned it, he has fulfilled his obligation; if he did not return it, he has not fulfilled his obligation, as he did not fulfill the condition, thereby retroactively invalidating the gift. The Talmud explains that Rava teaches us that a gift given on the condition that it be returned is considered a valid gift.