We could argue that the banker's recklessness and love of gambling foreshadow the financial problems which he experiences later in the story.
When the story opens, the banker is clearly a very successful man. He is hosting a dinner party for his friends and is wealthy enough that he can afford to make a hefty bet with the lawyer. In fact, he is described in the text as being a "self-confident millionaire."
Moreover, the banker is clearly very excited at the prospect of the bet because he can afford to win or lose:
The banker, spoilt and frivolous, with millions beyond his reckoning, was delighted at the bet.
By the time the bet is due to finish, however, the banker is living a very different life. Thanks to his recklessness on the stock exchange, the banker no longer has millions to gamble away on a bet. We see this clearly in the following quote:
He will take my last penny from me, he will marry, will enjoy life, will gamble on the Exchange; while I shall look at him with envy like a beggar.
By using the banker's confidence and recklessness to foreshadow his financial ruin, Chekhov demonstrates the importance of being sensible and cautious with money; otherwise poverty is sure to follow.