"You are forgetting your coat, sir"

At that moment, the overcoat lying on the chair caught the eye of the elder Jondrette girl. 

"You are forgetting your coat, sir," said she. 

Jondrette darted an annihilating look at his daughter, accompanied by a formidable shrug of the shoulders. 

M. Leblanc turned back and said, with a smile:— "I have not forgotten it, I am leaving it."

In this scene from the famous book "Les  Misérables," Jondrette is trying to exploit M. Leblanc (a.k.a. Jean Valjean).  It's clear from earlier sections that Jondrette has no interest in working for a living and that he resents those who have wealth.  Jondrette had purposely broke a chair and had his daughter break a window in order for his family to appear even more impoverished than they really were (he rejoiced when his daughter cut her hand in the process of breaking the window because it made them seem all the worse off).

Leblanc (Valjean) might know that he's being exploited or he might not.  But either way, he helps.  He gives them the money that he has on him and says that he will come back with more.

When he turns to leave, the older daughter of Jondrette notices that he has left his coat behind, much to her father's dismay.

But he hadn't forgotten it.  He had left it there on purpose.

To a scoundral like Jondrette, he willingly left his coat.
Did Jondrette deserve it?  No.
Had Jondrette even asked for it?  No.

Perhaps because Leblanc remembered when he was Jean Valjean, the convicted thief.  Released from prison, no one would take him in and no one would help him.  No one would offer him a room or food, even though he could pay.

Until he came upon a priest.  The priest welcomed him, fed him, gave him a room and wouldn't take any compensation.  So how did Valjean repay the priest?  By stealing his silver plates during the night and sneaking away under the cover of darkness.

Valjean didn't get far before being caught and brought back to the house of the priest.
Upon seeing the Valjean, the priest (who was already well aware of the missing silver) greeted him warmly and noted that he had forgotten to take the silver candlesticks as well.

The priest didn't have him thrown in prison, he gave the silver to the one who had stolen it.  The priest did not demand that he pay for his wrong; he forgave Valjean.

Having experienced grace, Valjean (Leblanc) now showed grace to another undeserving, lowdown sinner and left his coat behind.

Valjean had experienced the gospel.  Having been forgiven much, he now loved much.

Therefore I tell you, her sins, which are many, are forgiven—for she loved much. But he who is forgiven little, loves little."
(Luke 7:47)

"I have not forgotten it, I am leaving it."