when receive money from the customer this facility is available ( Bank transaction - Amount in INR - Optional in USD ) So that this gives me the correct conversion value. but when i enter the credit note there is no facility i mentioned above, so i can register USD value only therefore the value of exchange is changing in the p&l
Availability of foreign currency fields depends on context.
The feature appears when receiving money against a sales invoice for a foreign-currency customer. This is based on the customer, not the fact that you are entering a receipt. The purpose is so you can clear the receivable (which is in a foreign currency) by receiving an amount in your base currency (or other currency of the cash/bank account), with foreign exchange differences going to the appropriate account. A receipt for another purpose (no foreign customer) does not offer this feature.
A credit note does not involve the movement of money, so no equivalent amount is applicable. You are crediting a customer in the customer’s currency. Therefore, the feature does not appear. Exchange rates come into play only if you ultimately must pay money to the customer to clear their Accounts receivable balance. Then, the feature would appear on the payment form.