Use Google Flights to find a flight and then book it directly with the airline. Don’t use a website that isn’t the airline website to book a flight, ever. Looks like you have 2 options for those dates under 500€: https://www.google.com/travel/flights/search?tfs=CBwQAhojEgoyMDI0LTA4LTAzagcIARIDVEJTcgwIAxIIL20vMDE1NnEaIxIKMjAyNC0wOC0zMGoMCAMSCC9tLzAxNTZxcgcIARIDVEJTQAFIAXABggELCP___________wGYAQE
Thank you!