Am I still accountable for the money, given that I assigned someone to deliver the debt to its owner, but he refused and took it upon himself to deliver it, and I was subsequently unable to verify its delivery?

You must search for the owner of the right or someone who will pay on his behalf, so that your conscience may be cleared. If you despair of finding him, then give the amount as charity on his behalf. If you find him later and he approves the charity, then your conscience is cleared, and he will have its reward. Otherwise, you must return the amount to him, and you will have the reward of the charity.