iTunes card scam involving 3,500€

Review the information here for Gift Card scams.

About gift card scams - Apple Support

Another scam that is common are phishing scams where an email/message will attempt to scare you into clicking a link or calling a number to reveal your personal information. Once the scammer has your information, then your accounts can be compromised.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

One more thing is never let someone talk you into signing another account into your phone. They may promise you free apps or services, but in reality you have locked your device to them and you will not be able to remove it if they place your device in Lost Mode. At that point Apple cannot help, and the device will no longer be usable to you.

You may not get your money back from this Gift Card scam, but hopefully the information provided will help prevent you from being a victim of a future scam.