The situation is a great setup from Hugo. We have a man in a desperate situation, he is being crushed under his own wagon after an accident, and the wagon is slowly sinking in the mud, hastening the man’s death. Javert is on the scene and has quickly assessed that a jack is needed. One has been sent for, but it seems it will take 15 minutes for it to arrive.
Madeleine (Valjean) arrives on the scene and quickly comes to the same conclusion as Javert - the obvious solution is a jack. When he hears that it will be 15 minutes before one arrives he quickly ascertains that it will be too late, the man under the wagon will be dead.
Madeleine begins to offer an escalating award to anyone who can climb underneath and lift the wagon enough to rescue this man. With Madeleine desperately increasing the reward amounts, Javert turns to him and says he only ever met one man with that kind of strength - and that man had been a prisoner at the time. Here Javert makes intense eye contact and comes very close to saying outright “I know who you are, I know you were that man.”
Madeleine is in a bit of a bind here, and Hugo has set it up masterfully. As readers we now know that Madeleine is supposedly strong enough to do this himself, Javert may very well know his true back story, and Madeleine knows all of this. This sets the reader up as an observer to an unfolding dilemma: will Madeleine act to rescue this man and thus confirm Javert’s suspicions, opening him up to some degree of risk where his own future is concerned, or will he allow the man to perish, denying the code that he has apparently been following since his dramatic fork in the road conversion.
As we might have anticipated, Madeleine saves the man. His strength is enough to get the wagon to move a small amount and quickly others rush to help, lifting the wagon off of the man and saving his life. The chapter ends with us knowing Madeleine’s choice and seeing the man saved, but we have no idea how Javert will respond.