I reset the iPhone and it is locked on an unknown icloud

Hello, i need help! 🙂 Last month i was bought iphone 6, normally logged in with my Apple ID, and used it without any problems.
Today I had to sell it to a friend and when I reset phone ejects some totally unknown apple id (s*****@yahoo.com), i don't have "yahoo", i have gmail.

Then I've logged in to my icloud and in my devices is still standing phone, but can't any longer access it, wiped it from my mobile devices connect phone to the wi-fi, but still can not activate the phone.

I pray for help.