Responsa על בבא מציעא 60:21

Q. B claims that he gave A fifteen pounds to lend it on interest for two years and then give the principal and the interest to B's son if he should consent to marry A's daughter. B's son, however, refused to marry A's daughter and B wants his money back. A claims that he had originally accepted B's money as a dowry for his daughter, that he had taken possession of the gift for her, and that the money, therefore, belongs to her.
A. A dowry gift becomes the property of the donee only if the marriage takes place. Therefore, neither A nor his daughter has ever gained title to B's money, and A must return the principal plus the interest to B. Even if B expressly stipulated that he will forfeit the 15 pounds if the marriage does not take place, such a stipulation is considered an Asmakhta and is not valid. B, however, must pay A for his trouble in managing B's investments.
SOURCES: Cr. 86; Pr. 285; Mord. B.B. 615; Agudah B.B. 198.

Q. B claims that he gave A fifteen pounds to lend it on interest for two years and then give the principal and the interest to B's son if he should consent to marry A's daughter. B's son, however, refused to marry A's daughter and B wants his money back. A claims that he had originally accepted B's money as a dowry for his daughter, that he had taken possession of the gift for her, and that the money, therefore, belongs to her.
A. A dowry gift becomes the property of the donee only if the marriage takes place. Therefore, neither A nor his daughter has ever gained title to B's money, and A must return the principal plus the interest to B. Even if B expressly stipulated that he will forfeit the 15 pounds if the marriage does not take place, such a stipulation is considered an Asmakhta and is not valid. B, however, must pay A for his trouble in managing B's investments.
SOURCES: Cr. 86; Pr. 285; Mord. B.B. 615; Agudah B.B. 198.