The Sit-Mods on page 104 cover this sort of thing already. So it is a good idea. But I never like the idea of letting the players have this as currency.....
What do I mean by currency?
It is like a dollar bill they get to spend. And when they say they spend it then it is spent. They expect no arguments, and there is no denying them this. I don't like letting characters have free trumps. You have 1 flashback already and that is really enough. And a flashback can easily be +3 or even more once all modifiers are factored in.
So I say let the hero jump into the danger and then reward them greatly for taking the risks. But I am not a fan of them only jumping in because they know they still have a free bennie in their pocket.
Reward drama....... But leave the outcome uncertain.