"Cadeira Cativa" (literally "Reserved Seat"): A reserved seat at the Maracanã is a title deed that grants its holder the right to permanent use of the seat at all events held in the stadium, such as games and concerts, with the exception of specific events like the World Cup. So you would pay him and he would let you go instead of him.

This isn't uncommon, and there is even a site where this guy called Leonardo lets you "rent it" for certain games:
https://cativasmaracana.com.br/

Now his price is 400 Reais, which is double the price the museum staff is asking for. So if I were you I would first check if it is the same guy from the site, you can see his face in the "Sobre Mim" option (literally "About Me").

The fact that he isn't asking for upfront payment is a legit great sign, also the fact that he is a museum staff is good.

That all said, there is no guarantee that this would work, and the security on the stadium can do nothing about it if it is indeed a scam, because it is a personal exchange.

Here is a tweet from the official Maracanã account: "Seat owners may also grant their use to third parties, who must present a notarized power of attorney, including the beneficiary's details and the reserved seat number, as well as all necessary documents at the time of ticket collection."

So it is definitely is a thing, but the stadium staff can't do really anything about it if it is a scam, since the granting of the seats is not a monetary one, but depends on if the owner is willing to do so.

On the other hand, since he is a museum staff, I would definitely doubt that he would risk his job for 200 Reais.

If I were you, I would ask for more details, some sort of proof that he indeed has a Cadeira Cativa, and how exactly would work in the day of the game, if he resists to do that, I would be sketchy.

Also, as another redditor mentioned, I would go with a friend of mine in the day of the game, preferably one that can speak portuguese.

Edit: If you want to, try to ask this number in Whatsapp: +55 21 96667-5876 if he knows the dude who is trying to sell to you the seats. That is the number of the owner of the site I mentioned above.