So this "fight" has two brilliant aspects to D'Arby's fear and how Jotaro played him. One is deep spoiler territory so I won't go into that until Part 3 is over. The other however... think about the kind of person DIO is. "Oh ho? D'Arby, you wagered WHAT against Jotaro? Who said that was yours to wager?"
No matter the outcome D'Arby was a dead man walking if he took the bet.