aries & old man- Taos Akum is muttar. That means you can keep the money. Lets not get dramatic. Noone is going to lose their job over one mistake, especially today where everything is easily tracked by emails and electronic banking. They are not going to accuse you of anything or ask you why you kept the money. It was their mistake, not yours. I once received a duplicate deposit of about 2,000 into my account. While contemplating what to do, I noticed that there was a subsequent reversal of that deposit. No questions asked.
That said, it is definitely a kiddush Hashem to call them and return the money. BUT REMEMBER, you must stress that you are returning the money because the YWN CR directed you to… I mean because you are an orthodox jew and your religion stresses honesty etc.. Otherwise no kiddush Hashem was made.