Removing Activation Lock

I recently sold my iMac 27" and the buyer has moved away. He just contacted me and apparently, the activation lock is still on the iMac. I thought I removed it, but I guess I didn't. Anyway, I am trying to remove the Activation Lock so that he can recover and install fresh. Do I just simply delete the device from my account on icloud.com or do I need to remote erase it? Any help would be greatly appreciated.