What a mess. Don't give him your last four or any of that. I think you should offer him his choice of these two options:
1) Return/refund. He pays return shipping. It was unopened before, now it's opened. I think that's the price you pay for not having the knowledge ahead of time that would have made this an easy experience for your customer.
2) Continue to work with him to find an answer. I'm not sure what that is. But This option includes a promise that you'll stay engaged with him until the problem is solved or you have to go back to option 1.
1) Return/refund. He pays return shipping. It was unopened before, now it's opened. I think that's the price you pay for not having the knowledge ahead of time that would have made this an easy experience for your customer.
2) Continue to work with him to find an answer. I'm not sure what that is. But This option includes a promise that you'll stay engaged with him until the problem is solved or you have to go back to option 1.