You make a good point - in this instance, Voldemort is very much difficult to bargain with. However, I don't agree that that makes the problem impossible. For one thing, there may be solutions which don't require Voldemort's cooperation - e.g. silent transfiguration while stalling for time. For another, Harry can still get Voldemort's cooperation by convincing Voldemort that his current action is not in Voldemort's interests - for example, that killing Harry will actually bring about the end of the world that Voldemort fears.