The Indian Contract Act, 1872
34. When event on which contract is contingent to be deemed impossible, if it is the future conduct of a living person-
If the future event on which a contract is contingent is the way in which a person will act at an unspecified time, the event shall be considered to become impossible when such person does anything which renders it impossible that the should so act within any definite time, or otherwise than under further contingencies.
A agrees to pay B a sum of money if B marries C, C marries D. The marriage of B to C must now be considered impossible, although it is possible that D may die and that C may afterwards marry B.