now, I have a question, do you think the man is entirely at fault or someone should also be responsible?
Japan has some strict laws on gambling and its allowed to specific people under specific conditions. You cant gamble openly in Japan like you do in USA or Europe. The man shouldn't have used that money for gambling since in banking system you can easily be traced. So the man was at guilty since he did a crime knowing that he can easily be caught.
The truth of the matter is the man is guilty of using money that does not belong to him to gamble, this act is a criminal act, especially in Japan which has strict laws on gambling. The bank will trace the money to the man's account the only way he may escape jail is only if he is declared bankrupt, and given time to pay back even though it looks impossible for full repayments.
Moreover, in the OP it says the person concerned is willing to pay it in installments, regardless of whether the money is used for gambling or not, that person is basically wrong. Even though the person is using it for things other than gambling, he is still wrong, but in this case it is not only the person who receives the money that is at fault, because their employees are also at fault. How can the employee concerned send such a large amount of money but not be careful to make such carelessness, so in this case too I think both are wrong and they should not be imprisoned because after all, both are wrong. But if the two are not able to reconcile and continue through legal channels then of course, I think both have their respective faults so that this case is not worthy of being brought to trial.