law: Indian Contract Act, 1872 
  59. 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.
   Illustrations 
  (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.