Suppose the ritual to time travel lets in the Hounds? You have to survive them trying to nip the paradox in the bud, at the same time you break two Laws for the price of one. The law against time travel exists, so it isn’t impossible, it probably takes huge power and focus and risk to life. The Hounds attack before you time travel, so Eb knows either he or Harry will time travel, and that he can do it successfully as they have already survived the Hounds. Eb is the Blackstaff with permission to break the Laws, so why not? There are no consequences for him on this ocasion.