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 he should so act within any definite time, or otherwise than
under further contingencies.
Illustrations
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.