If you need to raise an invoice in Xero in a different currency you have two options:
Upgrading your subscription will definitely be worth it if you have bank accounts in different currencies, or if you raise invoices regularly that are in a different currency. Using the multi-currency function in Xero is straightforward and easy to set up. Read more over on their website here.
For a one off invoice, we would recommend using xe.com to find today’s FX rate, and working out the GBP equivalent of the Euro invoice. You will then need to state the FX rate and the Euro amount in the description box of the invoice.