United advanced 2-1 on aggregate after playing New York to a 1-1 draw Saturday and faces the Houston Dynamo in the conference final.
The Red Bulls appeared poised to take the lead in the 69th when they were awarded a penalty kick as United goalie Bill Hamid was given a red card.
In a wild series of events, the Red Bulls scored a goal that was taken back before D.C. backup keeper Joe Willis made a crucial save on the second attempt. Kenny Cooper put his initial penalty kick past Willis, but the goal was called off by the referee because the Red Bulls had entered the penalty area early.
Source: Associated Press