What do the recklessness and gambling habits of the banker foreshadow?

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.