The attack is directly on the notion of lying itself (not necessarily having anything to do with money).

The point is that it controverts the use of language, which is something necessary to us.  If everyone lies as much as they want, and does not feel bad about it at all, then we will be confused all the time as to what they do or do not actually believe.  No one would harbor the expectation that statements were trustworthy.  And therefore any given lie would no longer deceive.  So the idea of lying would no longer mean anything.  That is a contradiction in concept -- too much lying removes the possibility of the effectiveness of lying.  And in so doing it removes the ability to communicate directly at all, which would be a major loss.

There is no disputing the logic there.  The question is not whether the things he has defined exist.  The question is whether their existence actually matters.  That, he 'proves' by taking up 'autonomy' as his principal value, and applying a very simple version of the Golden Rule in a ruthlessly rigorous fashion.  If you were the extortionist, you would not want everyone to lie to you.  We need to more directly address the morality of extortion, rather than working around it by thwarting its effects.

Otherwise, we are not addressing morality as a community, we are using the criminal to justify our own misbehavior.  Once you do that with one category of misbehavior, it applies to a wide range of things you might forgive, including extortion of your own.  That is using a human being as a mere means to our own psychological comfort.

So he has foreseen your argument, and he addressed it explicitly, even down to the idea that you should not lie to prevent a murder.

That does not prove he is right, but it does mean that you need to make that more global decision for some other reason.