this problem is the reason why decision theories have to be non-deterministic. It comes up all the time in real life: I try and guess what safe combination you chose, try that combination, and if it works I take all your money.
Of course, you can just set up the thought experiment with the proviso that “be unpredictable” is not a possible move—in fact that’s the whole point of Omega in these sorts of problems. If Omega’s trying to break into your safe, he takes your money. In Nesov’s problem, if you can’t make yourself unpredictable, then you win nothing—it’s not even worth your time to open the box. In both cases, a TDT agent does strictly as well as it possibly could—the fact that there’s $100 somewhere in the vicinity doesn’t change that.
Of course, you can just set up the thought experiment with the proviso that “be unpredictable” is not a possible move—in fact that’s the whole point of Omega in these sorts of problems. If Omega’s trying to break into your safe, he takes your money. In Nesov’s problem, if you can’t make yourself unpredictable, then you win nothing—it’s not even worth your time to open the box. In both cases, a TDT agent does strictly as well as it possibly could—the fact that there’s $100 somewhere in the vicinity doesn’t change that.