On my part, it was the caprice of a well-fed man; on the lawyer's, pure greed of gold.
The banker here describes the actual impulses underpinning the bet. Although the bet is masked in the guise of a philosophical debate, at the core it is about money and image. The banker makes the bet because he can throw money around; the lawyer accepts it because he wants the two million rubles. This quotation demonstrates the slipperiness of materialistic impulses, how they can manipulate behavior even when the topic at hand is far more existential.
He will take away my last farthing, marry, enjoy life, gamble on the Exchange, and I will look on like an envious beggar…
In this quotation, the banker imagines how the lawyer will act upon receiving the two million rubles. On the precipice of losing the bet, the banker can only see the two million rubles he will lose, and therefore projects a smug triumph onto the lawyer. An examination of the books the lawyer has requested throughout his captivity suggests the years have not been easy for him, but the banker is so focused on his own finances that he cannot recognize this fact. His greed has poisoned his thoughts, leaving him blind to the truth.