This is what I found on forfeits over two legs; according to fifa.
A team sanctioned with a forfeit is considered to have lost the match by 3-0.
If the goal difference at the end of the match is greater than three,
the result on the pitch is upheld.
Never heard of a team actually doing this though. Would be interesting to see what Fifa would do if a team won 4-0 and decided to forfeit the 2nd leg so they could go through. I guess in most cases the money lost by not playing the game is a deterrent.
I've found more info from Uefa where if a team refuses to play a match then they forfeit that game and face elimination from the competition. I think this relates more to your question.
If a club refuses to play or is responsible for a match not taking
place or not being played in full, the Control and Disciplinary Body
declares the match forfeited and/or disqualifies the club concerned
in combination with the following fines
The fines range up to 1 million euro for doing so before the final.