If it was a legit mistake, he should have immediately communicated the error to you. If so, I would have no problem giving Hass back to the guy that dropped him, but I would make it effective in week 6. Issues like this have to be dealt with before kickoff on Sunday. If nothing was said before kickoff, I would probably have to say live and learn.
If this is a HTH league, what were the outcomes of the games involving these two owners? Would having Hass, or not having him, make a difference to their games?
In any event, I don't think you can go back and change the rosters and alter the outcome of the week 5 results.