I guess you learned the lesson of why you shouldn't open attachments from unknown sources. I hope you had a backup of your data that wasn't attached to the system and the time of infection. It is possible to remove the ransonmware, but there is no way of decrypting the files. https/www.pcrisk.com/removal-guides/9842-cerber-ransomware
If you want your files back, you will have to do whatever he wants. Even then, there is a pretty good chance you won't see those files again, these ransomware people couldn't care less what happens to you.