I'd leave it because there's no clear "right" course of action. Then I'd start to worry that the fact I even saw it might have significance in my decision. Then I'd figure that it's not my responsibilty to do anything just because I can, only if I will gain something in doing so (whether it be the satisfaction of helping someone or money to spend). At this point I'd start to analyze whether we really have a natural sense of justice or if it comes down to an individual's reasoning in situations like this. Then I'd get uncomfortable because the truth is that I wouldn't know what to do. Ideally I'd probably want to observe the money and see what course of action others who come in contact with it take and what follows. Then I'd become disappointed that "life" would make this obsessive endeavor impossible. I'd feel irritated that I'm hindered by the human condition - life itself would interfere with my search for the "answer" to this situation. Then I'd figure that life is so pointless, so fleeting, that I should be able to take the money because it bring a sort of primal excitement and joy. It'd be instantly gratifying and I could even indulge in saying it was fate that I receive the $40. Deep down I'd know, though, that this really isn't true. Is reason or emotion more human than the other? The only thing that's certain is that I have to make a decision. I'd refer to a logic trick: the concept of choosing to leave the money is not the same as it simply being left without my choice. I'd try to remove myself from the situation. I'd attempt to not be human, but only an intelligence. I'd let the event be recorded but not processed. This may be impossible though. Being human may mean I can't stimply let things occur - that I must always have an opinion. Things never are just the way they are. Despite all my reasoning, I've been compelled to type this analyzing a completely theoretical situation. I want someone to read this. I want some answer, or some sort of resonance with another human being. Or maybe this wall of text "just is" as well. This is likely only a brief analysis compared to all factors surrounding this situation. Personally though, I find it stupid that people even post shit asking questions like this because you have to be a total dumbass not to just realize and acknowledge the ambiguity in it, and in life. People are so fucking arrogant to think they have even the simplest of things figured out. You could even say that about how I just became aggressive in those last few lines. And after I'm done, this will sit on this forum, get buried, and never mean anything. Even if you've read this far, you likely won't remember this post or an of its contents after a reasonable length of time.
If you do remember it, it was because it was significant to you. You have related to what I've typed, negatively or positively. Isn't that astounding? To think that you are not alone.