Maicon Santos and Brek Shea scored first half goals for FC Dallas (12-6-7), which pulled within five points of the LA Galaxy for first place in the Western Conference.
Dallas played the final 10 minutes down a man following a red card to George John in the 81st minute.
Le Toux's second penalty kick came in the 86th minute after Jackson Goncalves took down Philadelphia's Gabriel Farfan in the box.
Adu, who signed with the Union on Friday, started as a midfielder and received a standing ovation when he was announced by the public address announcer.
Adu signed with DC United in 2004 at the age of 14, but hasn't played in MLS since 2007 with Real Salt Lake.
Santos gave FC Dallas a 1-0 lead in the 16th minute and Le Toux tied the match at 1 in the 34th minute on a penalty kick. Philadelphia's Justin Mapp was tripped in the box by Ugo Ihemelu, giving the Union the penalty kick.
But Shea - who has been in Philadelphia all week after picking up an assist for the U.S. Men's National Team last Wednesday in a 1-1 draw against Mexico - scored with stoppage time running out in the first half for a 2-1 advantage.
It was Shea's 10th goal, tying Vancouver's Eric Hassli for second in the league.(Copyright 2011 Philadelphia Union. Reprinted by permission)