We will say: The premise you have mentioned is wrong, because to doubt the miracle that will appear on the hands of the Imam is not inconsistent with belief in the person of the Imam in general. It is inconsistent with the fact that what is generally known and believed in is: he is this specific person or not. And such a doubt is not disbelief (Kufr). If such doubt were disbelief (Kufr), it would be disbelief (Kufr) even if he does not manifest a miracle, because before the miracle is shown, he doubted whether this person was the Imam or someone else. It would be consistent with his belief in the Imamate of the Imam in general, if he doubts his Imamate in general, and that is impossible.