A gift given on the condition that it is returned is called a gift TB Kiddushin 6

Daf TB Kiddushin 6 is the source for the well-known halakha known by anybody who attends the holiday of Sukkot services. Before we recite hallel, each davener takes a lulav and etrog and waves them in the six different directions. Because the Torah says “וּלְקַחְתֶּ֨ם לָכֶ֜ם בַּיּ֣וֹם הָרִאשׁ֗וֹן-On the first day you shall take” (Leviticus 23:40), the rabbis ruled that each person must own his/her lulav and etrog. He cannot borrow somebody else’s set.

Although everybody should come to the synagogue with his own set of lulav and etrog, not everybody does. How can a person who doesn’t own a lulav and etrog fulfill his obligation? The rabbis came up with a solution that when the owner of the lulav and etrog gives it to somebody as a gift with the condition he return it (מַתָּנָה עַל מְנָת לְהַחֲזִיר) after he has fulfilled the mitzvah. When he returns the lulav and etrog, the person has fulfilled this mitzvah.

The Gemara first records Rava’s statement.

“Rava says: With regard to one who says to another: Here are one hundred dinars for you that I am giving you on the condition that you return them to me, if he gave these one hundred dinars as part of a purchase, he does not acquire the item, as he has not given the seller any money. And similarly, with regard to a woman, if he gave her money for her betrothal on the condition that she return it, she is not betrothed. If one gave money in this manner for the redemption of his firstborn son, for which a priest must receive five sela, his son is not redeemed.

“If one does this with regard to teruma, i.e., he gives produce to a priest as teruma on the condition that it will be returned, he has technically fulfilled his obligation of giving. Once he gets the teruma back it belongs to him, as he is the original owner, and although it is prohibited for him to partake of it, as he is a non-priest, he may sell it to a different priest. But it is prohibited to do this, i.e., give teruma in this manner, ab initio, because this priest receiving the teruma appears like a priest who assists at the threshing floor, as he presumably agrees to this arrangement in return for some gain.” (Sefaria.org translation)  

Then the Gemara shows his inconsistency.

“The Gemara asks: What does Rava maintain? If he maintains that a gift given on the condition that it is returned is called a gift, this should apply not only to teruma but even to all the other cases, i.e., it should be considered a valid gift in all of the above cases. And if he maintains that a gift of this kind is not called a gift, then even with regard to teruma it should not be considered a legitimate form of giving.

“And furthermore, Rava is the one who says: A gift given on the condition that it is later returned is called a gift. As Rava said that if one says to another on the first day of the festival of Sukkot: Take this etrog on the condition that you return it to me, and the recipient takes it, recites a blessing over it, and returns it, he has fulfilled his obligation, despite the fact that one must own the etrog he uses for the mitzva on the first day of Sukkot. And if he does not return it he has not fulfilled his obligation, as he gave him the gift only on the condition that it would be returned. This indicates that in the opinion of Rava, a gift that is given on the condition that it is returned is considered a gift.” (Sefaria.org translation)  

Rav Ashi corrects Rava’s first statement and is corroborated by Rav Huna Mar, son of Rav Neḥemya.

“Rather, Rav Ashi said: In all of these cases the gift is acquired, except for the betrothal of a woman, because a woman cannot be acquired by means of symbolic exchange. Rav Huna Mar, son of Rav Neḥemya, said to Rav Ashi: We say this in the name of Rava in accordance with your opinion, not in accordance with the previous ruling.” (Sefaria.org translation)  

When I speak to the couple before the wedding and tell them all the halakhot surrounding the ring the groom will give the bride underneath the khupah to the effect kiddshin, I remind him that he has to own the ring without any conditions. If there’s some family heirloom he wishes to use, the owner has to give it to him outright. He cannot give it to the bride on the condition that she return it to the original owner. The ring is hers to keep. However, if she chooses of her own free will to return it to the original home she can because it is hers free and clear.