I am tempted to ask: What would you think if the roles were reversed, and you were the one who took the $100 from a stranger and what you would hope the stranger's response would be if it was discovered that you took the money and then gave it back? But I will try a different approach. Taking the case as you describe it: I suggest you would be within your rights (and not wrong) to report this as a theft and the stranger would then face whatever penalty the law specifies for petty theft, but I think you would also be within your rights (and not wrong) to not report this. Imagine that the stranger changed his or her mind within just two minutes and apologized profusely to you, perhaps even offering you the lottery ticket he just bought (and did not steal) and this gives you a finite chance of winning millions later in the week. Still, a theft or stealing has taken place even it the funds are returned. Imagine that the stranger stole millions from a pension account for hundreds of vulnerable, retired people. Even if never detected and the money is returned with interest after a week, the person wrongfully took possession of something that he or she was not entitled to. We might even consider a more dramatic case: imagine a stranger puts a poison in your coffee that you do not detect and it will kill you after 24 hours. After 23 hours the stranger repents. He happens to have a cure that will remove the poison from your system, leaving no trace and, instead will actually provide you with life-extending vitamins. Imagine the stranger is able to put the cure in your sparkling water and the only difference you notice is a renewed sense of vitality and cheer. If discovered later (perhaps there is a reliable informant who discloses what took place or the stranger confesses or the poison and cure-maker comes forward), I think most of us would think (rightly) that the stranger was guilty of attempted murder and that the state has a legitimate reason to take action in the form of punishment (or perhaps committal to a psychiatric ward until one may be certain that the stranger has been cured of whatever pathology drove her or him to put you through this threatening experience).
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Suppose a stranger steals 100$ from me, then has second thoughts and puts it back, before I ever come to look for or need the 100$. If discovered, should he be punished? Why or why not?