maiciy, do not worry. Once the device is received it will be wiped. It is crucial that you disable Find My iPhone. Here's how to do it.
You can disable the Find my iPhone setting even if you already sent the phone back to us. Please follow these steps to disable the Find my iPhone setting. From an internet connected device, open a browser and go to icloud.com.
1. Log in to iCloud.com with your Apple ID.
2. Click or tap Find My iPhone.
3. Click or tap All Devices.
4. Click or tap the device that needs to have the Find My iPhone disabled.
5. Click or tap Remove from Account.
6. Click or tap Remove.