The banker decided to wager the bet in order to prove a point and to fulfill his desire of witnessing a greedy man's downfall (the lawyer).
This situation can be considered as a matter of personal justice according to the banker's description of the lawyer as a "well-fed man" with "pure-greed of gold"; the banker didn't want to see the lawyer get away from the hands of what they thought of as justice.
Even though the banker hesitated a bit in their decision, they ended up compelling themselves to believe they did the right thing.