Is Voldemort familiar with logical syllogisms? If not, it should be possible for Harry to trick him by saying something that seems to imply something else, without actually confirming the second thing as true, a la Chapter 49:

Harry kept his face steady. "I was looking up some facts about the Patronus Charm earlier," he said. "According to The Patronus Charm: Wizards Who Could and Couldn't, it turns out that Godric couldn't and Salazar could. I was surprised, so I looked up the reference, in Four Lives of Power. And then I discovered that Salazar Slytherin could supposedly talk to snakes." (Temporal sequence wasn't the same as causation, it wasn't Harry's fault if Professor Quirrell missed that.) "Further research turned up an old story about a mother goddess type who could talk to flying squirrels. I was a bit worried about the prospect of eating something that could talk." (emphasis mine)

One possible example proposed in a review on fanfiction.net (and the one that set me on this train of thought in the first place) is, "If you kill me, the world will end." Since the world will end no matter what, the consequent is guaranteed true, making the content of the antecedent irrelevant due to contrapositive shenanigans... but Voldemort doesn't know that, and it sounds like the end of the world is dependent on Harry's death.