Indian Contract Act, 1872
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 the 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.