I once got a deposit in my chase account by mistake, the rav was trying to convince me that I should make a "kiddush hashem" and return it so I was explaining the rav that the guy will never know that I had it & returned it some rep will just take the money & put it into his account & story closed, so the rav asked me how much are we talking about so I said 5k! He was like oh ok in that case...
Anyways in the end (2 weeks later) chase caught onto it & they took back the money.
Ps they even took back the interest (some 30 cents) that I earned on this money....!!