Section 58 in The Indian Contract Act, 1872
Title: 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.
Title: Application of payment where debt to be discharged is indicated.
Where a debtor, owing several distinct debts to one person, makes a payment to him, either with express intimation, or under circumstances implying, that the payment is to be applied to the discharge of some particular debt, the payment, if accepted, must be applied accordingly.
(a) A owes B, among other debts, 1,000 rupees upon a promissory note which falls due on the first June. He owes B no other debt of that amount. On the first June, A pays to B 1,000 rupees. The payment is to be applied to the discharge of the promissory note.
(b) A owes to B, among other debts, the sum of 567 rupees. B writes to A and demands payment of this sum. A sends to B 567 rupees. This payment is to be applied to the discharge of the debt of which B had demanded payment.
Title: Application of payment where debt to be discharged is not indicated.
Where the debtor has omitted to intimate and there are no other circumstances indicating to which debt the payment is to be applied, the creditor may apply it at his discretion to any lawful debt actually due and payable to him from the debtor, whether its recovery is or is not barred by the law in force for the time being as to the limitation of suits.
Title: Application of payment where neither party appropriates.
Where neither party makes any appropriation, the payment shall be applied in discharge of the debts in order of time, whether they are or are not barred by the law in force for the time being as to the limitation of suits. If the debts are of equal standing, the payment shall be applied in discharge of each proportionally.
Title: Effect of novation, rescission, and alteration of contract.
If the parties to a contract agree to substitute a new contract for it, or to rescind or alter it, the original contract, need not be performed.
(a) A owes money to B under a contract. It is agreed between A, B and C, that B shall thenceforth accept C as his debtor, instead of A. The old debt of A to B is at an end, and a new debt from C to B has been contracted.
(b) A owes B 10,000 rupees. A enters into an arrangement with B, and gives B a mortgage of his (A's) estate for 5,000 rupees in place of the debt of 10,000 rupees. This is a new contract and extinguishes the old.
(c) A owes B 1,000 rupees under a contract. B owes C 1,000 rupees, B orders A to credit C with 1,000 rupees in his books, but C does not assent to the arrangement. B still owes C 1,000 rupees, and no new contract has been entered into.