You've actually made it an easier problem for me, though, because I regard my alternate selves as other people.

How do you peak the threshold, decide that [$10000] < [decision threshold] < [your life]?

If it were possible for me to make a deal with my alternate self by which I get a few thousand dollars, I would obviously surrender my $100. As it isn't possible, I see little reason to give someone otherwise destined to be forever causally isolated from me $10000 at the cost of $100. I wouldn't keep $100 if it meant he lost $10000, either. I probably would keep the $100 if they lost less than $100. If my alternate self stood to gain, say, a million dollars, but nothing if I kept my $100, then I probably would give it up. But that would be as a whimsy, something to think about and feel good. But the benefit to me of that whimsy would have to be worth more than $100.

The pattern behind my choices is that the pain experienced by my alternate self (who, recall, I consider a different person) in any of these cases is never more than $100. I think this is the most we can expect, on average, of other intelligent beings: that they will not inflict a large loss for a small gain. Why not steal, in that case? Because there is, in fact, no such thing as total future causal isolation.