How to unblock icloud

good morning everyone.When i tried to sell my iphoneX i’ve met a rogue.He ask me to enter his icloud to check if its work,so if its work then he’ll be buy my phone. But result is when i entered hiss icloud he change the password,and ask me to pay money for him if i wanna exit his icloud.I have all documents for my iphone,Please help me to unblock it