Responsa על בבא קמא 204:5

Q. A lent money to a Gentile. Part of this money belonged to B, but the Gentile was unaware of it. A collected the loan but refused to give any part of it to B saying: "Go and demand your money from the Gentile."
A. A must pay B the money coming to B from the Gentile for the following reasons: a) We assume that A became a trustee for B's part of the loan and, therefore, must collect it for B, for how else did B expect to get his money back?; b) A caused a definite loss of money to B and must compensate him for such loss; c) the Gentile became obligated to pay money to B, even though he did not know that part of the money loaned to him belonged to B. Therefore, when the Gentile paid off his debt to A, he gave to A the money he owed to B, and A must give that money to B.
SOURCES: Pr. 254.

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.