Hi. I wanted to make a fan game, and I requested permission via email to use a character. The copyright owner allowed me to use his character, as long as my game is not commercial. Now, I have seen that he is considering selling his rights to a publisher or a bigger studio. If he sells his rights and I release my game, could it be taken down by the publisher, altough I got permission from the previous copyright owner ?

The short answer is that you can always be threatened with court action, and that can end up expensive, even if they have little chance of winning the case. So if that worries you, give up now.

Assuming that's not an issue, the situation you're in is awkward. Technically he has given you a licence for a certain kind of usage, and yet he will probably be selling an exclusive licence to the publisher or studio. That essentially makes him the one in the wrong as he's selling something that he no longer really has. If you can prove this, you might be able to defend your position and continue with your game. Practically, however, you might just find yourself on the end of threats from the company's legal department, who might claim that your permission via email is fake, or they may even be able to claim that the contract is invalid since there was no consideration from your side. ("Consideration" here is a legal term implying payment or some other concession made by you to secure the deal.)

Fan games are a troublesome field to deal with. The only way to be safe is to steer clear.

I have seen that the owner said that selling his rights to a company is an option among others, altough he doesn't find it very appealing. It seems extremely unlikely for this to happen, based on the feedback that fans gave him. In this case, I guess it is safe to work on the fan game.