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 agreement 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 agreement. B still owes C 1, 000 rupees, and no new contract has been entered into.