Is money considered a trust (amanah) for the wife, and must it be returned to her after the husband requested it and was given it with the consent of the wife, who now claims she consented under duress?

You have done well by safeguarding the trust and not neglecting it, and this is a Muslim's duty. If you are certain that your uncle's wife authorized the payment of the deposit to her husband, then there is no sin upon you and no liability for the amount, because you did not fall short in safeguarding or returning the trust. As for her claim of coercion, it has no relation to your action; it is a problem between her and her husband.