Exchange Rate for Particular Invoices

I would like to have an option to fix exchange rate for particular invoice at the time of creation of invoice. In case not fixed it should take the default exchange rate as per the system.
This is required to fix the sales in the FC and local currency. We need to file monthly sales in local currency to GST and later sometimes with change in default exchange rate the sales values change and create confusion.
Please help to create this option.

Secondly we are inputing exchange rate as 1 INR = 0.131 Euro. Instead we will prefer an option to put the currency directly like 1 EURO = 77 INR. This will bring the exact values that we need.

There is no exchange rate associated directly with an invoice. The invoice is denominated in the currency of the customer or supplier and does not change. Exchange rates come into play when Accounts receivalbe or Accounts payable are converted to your base currency for the balance sheet. That cannot be fixed because, if the exchange rate changes, the value of the asset or liability in your base currency will fluctuate.

When money is paid or received against an invoice, you do have the ability to enter the equivalent amount in the customer or supplier’s currency to correspond with the amount entered in the currency of the cash or bank account. This forces calculation of an exchange rate for that specific transaction and ensures that the amount posted against the invoice in its native currency remains fixed. That keeps invoices from becoming paid and unpaid as rates float.

Exchange rates in the program always show base currency first. If your base currency was changed to Euro, it would show as you want. But the rest of your post does not make sense. An exchange rate of 1 INR = 0.131 EUR is nowhere close to 1 EUR = 77 INR (which is equivalent to 1 INR = 0.01298 EUR).