Hi Nikesh,
Receipt entry in your branch will be normal :-
Bank A/c. Debit - 15,000/-
To Customer A/c. - 15,000/-
Now, you want to pass entry for credit note to be issued. Means, you are not actually paying that amount to Delhi branch but you want to pass only adjustment entry. Hence, in such entry Bank account will not be involved. Similarly, Customer account will not come again here. Your entity is following practice of branch accounting and hence in Bal Sheet, there must be inter branch balances. Hence, in my opinion, entry for credit note to be issued will be :-
Your Branch A/c. Debit - 5,000/-
To Delhi Branch A/c. - 5,000/- (credit bal - since payables)
Other views are welcome.