Explain the Underground Man’s final act of revenge against Liza. What is the symbolic significance of throwing the money at her?

Throwing the money at Liza is one way the Underground man
seeks to establish some type of power in an existence that lacks it.  After spinning
tales of how superior he is to her and how her life will be condemned to misery if she
does not leave prostitution, the Underground Man has made himself to be a hero, if only
for a moment.  Yet, when Liza arrives at his home, it is apparent that she understands
his weaknesses and how he is incapable of love.  After seeing his home and seeing who he
really is, she understands him.  She recognizes that although he needs love and needs
some type of redemption badly, she also recognizes that he has become a product of the
world around him.  He cannot be saved for this reason.  Someone would have to literally
sacrifice themselves to do so and even this is no guarantee.  The same degrading that he
spoke to her would happen if she remained in prostitution is the same that would happen
to anyone if they fell in love with the Underground man.  At this moment, when she fully
understands how he is condemned to being a product of his own environment, she
transcends her with her embrace.  This jarring and powerful breaking of environmental
context and contingency is seismic for the narrator.  His only way to reestablish some
level of power and some level of normalcy is to bring back the environment that he hates
so much and throw the money at her.  Yet, in the end, she has become the figure that
proves to us that there can be saving if individuals understand the need and demand to
not become a part of the world that surrounds and corrupts them.  She has understood
this, while seeing through the narrator in the process.