The Union (7-13-7), which won at Houston on May 29, were officially eliminated from playoff contention after Seattle beat Toronto FC earlier in the day.
Andrew Hainault scored for the Dynamo (7-14-6), who fell to 2-9-3 on the road.
Le Toux tied it with his 13th goal of the season. The French midfielder got a cross from Roger Torres, corralled the ball on his foot and then sent a one-timer past goalkeeper Pat Onstad.
Le Toux is tied for second in the league in goals with Toronto's Dwayne De Rosario and New York's Juan Pablo Angel, just one behind Edson Buddle of the L.A. Galaxy.