But it's already out of his hands now, yes?

[Unless he's missing something here.]

He can't obtain it again without gaining your agreement? What's to stop you from making this bargain yourself? Getting them for yourself?

[The thrust of her argument makes perfect sense to him. Yes, she absolutely should want to destroy something so evil. It's the framing of this as the "only way" that has him highly suspicious.]

Why could you not be the one to hold them until the disentanglement method is ready? If he's truly serious about this, that shouldn't be an unreasonable request.