My neighbour is rich. I want in on that
Solution 1:
First, usurp his liege's title. This will cause him to become independent and, more importantly, gives you a de jure claim on his county.
Press the de jure claim. You need to do this instead of holy warring because holy war will only take his title (and he'll move to another court).
When you win, you'll vassalise him. Revoke his title. Hopefully, since he hates you, he'll refuse. This will allow you to imprison him without tyranny.
Once he's imprisoned and his title has been revoked, banish him. You'll get all his money.