law: Indian Contract Act, 1872 
  50. Performance, in manner or at time
  
  prescribed or sanctioned by promisee 
  The performance of any promise may be made in any manner, or at any
  
  time which the promisee prescribes or sanctions.
   Illustrations 
  (a) B owes A 2,000 rupees. A desires B to pay the amount to A's
  
  account with C, a banker. B who also banks with C, orders the amount to be
  
  transferred from his account to A's credit and this is done by C. Afterwards,
  
  and before A knows of the transfer, C fails. There has been a good payment by
  
  B.
  (b) A and B are mutually indebted. A and B settle and account by
  
  setting off one item against another, and B pays A the balance found to be due
  
  from him upon such settlement. This amounts to a payment by A and B,
  
  respectively, of the sums which they owed to each other.
  (c) A owes B 2,000 rupees. B accepts some of A's goods in
  
  deduction of the debt. The delivery of the goods operates as a part payment.
  (d) A desires B, who owes him Rs. 100, to send him a note for
  
  Rs. 100 by post. The debt is discharged as soon as B puts into the post a
  
  letter containing the note duly addressed to A.