Of course the man is obligated to return the money. I don't know what the law is in Japan, but in few countries you cant even take the money you find on the street. In many countries, if you find any money or it falls into your hands in some other way, you must refer it to the authorities. Only if their owner is not found for a year, you can keep them for yourself. In this case, there is no doubt that this man spent not his money and must return it.
Agreed with your arguments. But why no action being taken against the bank? Bankers are the ones who receive the fattest paychecks and how can they make mistakes like this? For such big amounts, don't they need multiple levels of verification? And I am even more surprised that this happened in Japan. That country is well known for work efficiency, and something like this is unheard of. Anyway, the individual in question may go to jail, but I don't think that the bank will get back any of the money (since it is already used up).
I don't really understand why you concluded that it was a mistake of the bankers?
In my opinion, it was a mistake of the person ordering the transfer. Of course, this should not be the case and the consequences should be faced. However, the banking system works differently from blockchain and this difference is the reversibility of transactions and the complete absence of anonymity.
I am not sure if a bank employee or an employee of some other institution that ordered this transaction made a mistake, but do you think the consequences should be borne by any of these people?