Wrong Keys Given

The user has until seven days after the Giveaway to give you the proper game. If not, mark it not received.

Contact the giver and inform him/her about the situation. They have 7 days from the end of the giveaway to provide the correct game. If you haven't received it after a week then mark it not received.

just post on the giveaway page. chances are he doesnt have the key anymore since it probably was given on another giveaway, though.

if you are unhappy with that outcome, then just mark it not received when you are able.

if you feel it was an honest mistake and you are content with receiving the game you received, you can state its an acceptable alternative on the page and he will be able to open a support ticket to have the giveaway changed to the game you received. then you can mark the correct game as received and he wont have the undelivered game on his profile.

It happened to me a few weeks ago, on the giver side. I made a copy-paste error and sent unwillingly a key for game Z to the winner of game X. He added me on Steam and warned me about that, I gave him the good key, problem solved.
TL;DR It can be a really innocent mistake from the giver, get in touch with her/him.