Voldemort has an absolute truth oracle, or at least a sufficiently good approximation thereof, available too him. If Harry needs his wand to teach V one of his secrets, make him say so in Parseltongue. If H does demand his wand, make him state whether he intends to use it for anything but demonstrating a secret.

The wonderful thing here is that this gives all kinds of opportunities for V to screw up without realizing he's screwing up. PT is a secret for which H needs his wand. H is, in a sense, demonstrating PT. Unless V was very careful about making H state his exact and full intentions, we could have had a plausible reason for H to have his wand. There's really no plausible reason for V to just let him have it, though; disarming him (there's even a spell which does exactly that, and one would hope his followers know it...) costs nothing but a small amount of time, and gains V potential defense against "a power he knows not"... the existence of such things being the whole reason V didn't just have H killed immediately!