Opting out of an overdraft service does not mean your bank is prohibited from paying an item against insufficient funds. Although I personally would bounce NSF items authorized by people who have opted out (same revenue for me either way), most banks just continue to pay or return on an ad hoc basis.
If this was an electronic payment, your customer would have to say the charge was unauthorized or the authorization had been revoked in order to claim harm. He's not saying that. So, the payment was appropriate.