Rav Nachman bar Rav Chisda expounded: If one said to another, “This esrog is given to you as a gift, and after you (his death) it shall be given to So-and-so,” and the first recipient took it and fulfilled his obligation with it, this will be a point of dispute between Rebbe and Rabban Shimon ben Gamliel (for according to Rebbe, who maintains that the one who has the rights to the produce is not regarded as the owner of the object, he will not have discharged his obligation, for the esrog is not his; according to Rabban Shimon ben Gamliel, it is regarded as his, and he will have fulfilled his obligation).
Rav Nachman bar Yitzchak asked him: The dispute between Rebbe and Rabban Shimon ben Gamliel is only if the acquisition of the produce is like the acquisition of the capital or not, but here, if the first recipient cannot discharge his obligation with it, for what other purpose was the esrog given to him! Rather, it is clear that everyone holds that the first recipient may properly discharge his obligation with it; the argument would be regarding a case where he sold it or ate it.
The Mefarshim ask: According to Rebbe, who holds that the one who has the rights to the produce is not regarded as the owner of the object, how can the first recipient discharge his obligation with this esrog? It is not regarded as “lachem” – completely his, so what difference does it make that the donor intended for him to fulfill his mitzvah?
The Ritva explains that since the donor’s intention is that the first recipient should discharge his obligation with it, it must be that he gave him the guf (capital) and the peiros (the produce) completely; however, he stipulated that he, after he has discharged his obligation with it, must give it over completely to the second person. This would be similar to a gift that was given on condition that it is returned to him.
Reb Shmuel Rozovsky explains as follows: It is evident from the Gemora above (136b) that even according to Rebbe, it is possible to give someone rights to the produce that will be regarded as an acquisition of the guf. For the Gemora said that a father, who retains the rights to the produce in a case when he gave away the land to his son, since it affects him personally, he made sure to keep certain rights to the land along with the produce. So too in this case, it is evident that the donor intends to give the guf of the esrog along with the peiros; accordingly, the recipient can discharge his obligation with it.