Voldemort didn't reply to Theo's last message. While He was confident Theo would deliver the message, He wasn't putting much stock in the man's acquiescence and certainly wouldn't trust him to do His bidding in more ... complex tasks. At least not yet. Still, the Head of the Department of Mysteries could be a useful ally. Voldemort would be careful on the tasks He would give the man and would have to find a suitable reward, should he fulfill this first task.