Does he deserve -rep?

It depends on the circumstances. If he had offered you a deal that involved you expecting to receive the full game, and the beta key does not give you that privilege after the beta ends, then he has not delivered and thus he did not uphold his end of the trade = negative rep. However, if he gave you exactly what you asked for, and you simply found out later that what you asked for didn't give you what you wanted, then it's your own fault. You will know which one is correct.

Minus until he tries to compensate you

If I were you I'd talk to him and explain that you gave him a full game for a beta which will expire and that makes it an unfair trade and that you'll have to give him a -rep.
Keep it civil and kind until there's no other option.
If he compensates you be sure to mention that he was really cool about it.
But never forget, you can never know who's on the other side of that chat- it might be a kid who knows nothing and is just getting started but it could also be a person who knows exactly what he's doing and he's trying to take advantage of you for being nice and understanding...

If their thread/offer didn't mention that it was a beta key, ask them for something else to compensate you imo. If they refuse -rep them.

Is it allowed trading beta keys in sg?

I would probably put a -rep. Excuses like this don't really impress me and he is probably screwing with you...

1. You & he deal for ( X ) game

2. He give you ( Y ) game

3. You say about his mistake !

4. He says i`m sorry ! . I dont have ( X ) game !

5. HE DESERVE - REP

End of story !

He's just acting dumb. Do you really think he doesn't know what he's giving you?

In my opinion, of course we must negotiate.
Tell him what he has done, and you will give him -rep, but be open to negotiation.
If he tell you that he is sorry, ask him for compensation. He has received something, and he can't return it, so that you must also receive something from him.

If he refuses to cooperate and refuses to compensate, then give him -rep.

If he is legitimately sorry, he will try to compensate and does not deserve a -rep, but if he is not sorry he will not compensate, and should receive a -rep.

Always assume the worst, in this case that he's an evil mastermind who wants to steal your games!
Thereby, I'd have given him -rep

I've tried to contact him. Until yet he didn't respond. Therefore I choose to leave a -rep until he decides to compansate the bad trade.
Thanks to all of you sharing your thoughts about this.
<3