Responsa for Shevuot 76:8

Q. A sold his house to B and undertook to settle with the abutter so that the latter would not take the house away from B. The abutter, also, told B that having no money he did not want to buy the house. B, however, failed to bind the abutter by a kinyan. A borrowed jewelry from his wife and deposited it with C stating: take formal possession of this jewelry on condition that if I fail to settle with the abutter it will belong to B from now on. B, on the other hand, deposited twenty^*Cr. reads: “two;” Mord. reads: “four;” L. and Tesh. Maim. read: “twenty.” marks with C as a guaranty that he would pay the price of the house and that he would not change his mind. After the transaction was concluded, however, the abutter obtained money, paid off B, and took away the house; B, therefore, demanded of C that he turn over to him the valuables A had deposited with him. C told A in the presence of witnesses of B's demand and A replied that he should give the valuables to B "since it is legally coming to him". Is B entitled to the valuables?
A. A gave the valuables to C in order that he deliver them to B should a certain condition not be fulfilled. Such a transaction is called asmakhta and is not binding since it was not made before an authoritative court. When A finally told C to deliver the valuables to B, he was acting under misapprehension that they were due him legally, as his statement indicates. His order, therefore, was not binding and B should return the valuables to A.
SOURCES: Cr. 290; L. 309; Mord. B. B. 324; Tesh. Maim to Kinyan, 3.