Some conceptions of free will, compatibilist ones mainly (though perhaps not all compatibilist ones) see human minds as something like decision making machines. If the machine of your decision making apparatus outputs a choice given a particular environment, then, for all intents and purposes, that choice is "free" - and this remains true even if that decision making apparatus operates deterministically.
Thus, if Harry Potter goes back in time and does something, then even if we know that he was guaranteed to do that thing, it was still a valid and "effectively free" output of his decision making apparatus. It was still HIM that made that choice.
