iPhone activation lock

Hello. My question will be that I recently decided to buy my girlfriend with Iphone 7 money earned. I bought it from a man by hand. When inspecting the phone, everything was fine, iCloud is clean, everything works, no problem. When I came home, I decided to dump it so that it would be like new. But then I saw that there was an activation lock on my phone. I started calling a man, he says he doesn't know anything, he was normal and working. I can't prove something to him. Could you please tell me if I can solve my problem in some way?