The Indian Contract Act, 1872
58. Alternative promise, one branch being illegal-
In the case of an alternative promise, one branch of which is legal and other other illegal, the legal branch alone can be enforced.
A and B agree that A shall pay B 1,000 rupees, for which B shall afterwards deliver to A either rice or smuggled opium.
This is a valid contract to deliver rice, and a void agreement as to the opium.