Kiddushin 46B

ושמע מינה: מעות בעלמא ⁠– חוזרים.

And conclude from it that generally, money that was given for a betrothal that did not take effect is returned. It is not viewed as a gift, but as a loan that must be repaid. This is evident from the fact that all the dates except the last one are considered a loan that must be repaid.

איתמר: המקדש אחותו, רב אמר: מעות חוזרים. ושמואל אמר: מעות מתנה. רב אמר: מעות חוזרים ⁠– אדם יודע שאין קידושין תופסין באחותו, וגמר ונתן לשום פקדון. ולימא לה לשום פקדון! סבר לא מקבלה.

§ It was stated: In the case of one who betroths his sister by giving her money, Rav says: The money of the betrothal must be returned by his sister, as this betrothal does not take effect. And Shmuel says: This money is considered to be a gift that she may keep. The Talmud clarifies their respective opinions. Rav says: The money must be returned, since a person knows that betrothal does not take effect with his sister, and he decided to give the money to her for the sake of a deposit. The Talmud raises a difficulty: And let him explicitly say to her that he is giving her the money for the sake of a deposit. The Talmud answers: He thought she would not accept it from him.

ושמואל סבר: מעות מתנה ⁠– אדם יודע שאין קידושין תופסין באחותו, וגמר ונתן לשום מתנה. ונימא לה לשום מתנה! סבר: כסיפא לה מילתא.

And Shmuel maintains: The money is considered to be a gift, because a person knows that betrothal does not take effect with his sister, and he decided to give the money to her for the sake of a gift. The Talmud again raises a difficulty: And let him explicitly say to her that he is giving it to her for the sake of a gift. The Talmud answers: He thought it would be embarrassing to her and she would refuse to accept the money. He therefore attempted to give it to her by an alternative method.