Eh, I won't judge you if you don't return it. If you truly need it that is, and you're not really to blame if you were to ever run with it, simply because it was an error on their part and not necessarily yours. I would probably not return it if I were in your shoes, especially if the said exchange stays silent and doesn't demand a return. If they demanded, I might give in since I'm not particularly at a difficult spot, but if they don't, I don't see a reason why I should.

Heck, you can even negotiate about retaining maybe half the amount. I'm pretty sure you have the advantage in terms of negotiation power here since you have the funds, while they don't necessarily have the right to argue due to their mistake. It's really up to your moral compass on how much you're willing to return.