In Odoo 17.4 the system will show you the currency exchange rate automatically on the Invoice without having to do anything.

1. You can configure your currencies and their rates:

2. Then when you create an invoice the exchange rate will be visible:

3. Also, when you click on Journal Items tab and add the Amount in Currency column you will be able to see the credit and debit in the main currency of the database, and the amount in currency will show the amount of the invoice in the invoice's currency: