Just claim you were hacked, and that you never traded. If you've already recalled, then you've got some evidence to show it like you were locked out of your account, possibly hacked. Blizzard will see this and probably let you keep the account.
That's of course assuming he actually follows through, because I doubt he would want to end up with no accounts. Just be cool with him and let him have his old account back. If you aren't a total douche you should be fine and he will calm down.