At night at the barricade, who do the men discover is there as a spy?

In "Saint-Denis" book 12 (Corinth), the student revolutionaries build a barricade out of everyday items. Others join the group. When it is done, they wait for the attack. Gavroche is the one who recognizes Inspector Javert among the new recruits. Javert has infiltrated the ABC Society to gain information. As soon as Gavroche identifies the spy, the students tie Javert up and plan to execute him.
In "Jean Valjean" book 1 (War Between Four Walls), Jean Valjean joins the ABC Society at the barricade. He sees Javert tied up inside the bar. As the fighting goes on, Enjorlas realizes that the barricade is falling. He foresees the enemy overtaking them soon, so orders Javert's execution to happen immediately. Jean Valjean volunteers for this, but instead fires his gun into the air. He lets Javert go free. This moment has a big impact on Javert.