Section 58 of Contract Act, 1872
Alternative promise, one branch being illegal
In the case of an alternative promise, one branch of which is legal and the 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.