It's a scam. Here's how it works.
You will be sent a foreign bank check for more than you're supposed to get. You are
supposed to deposit it and wait for it to clear. Since it looks legit your bank will
credit your account for the amount before the check clears. You will think you have
the money in hand but you don't. Then you transfer the difference through some
untraceable, non-reversible method, probably Western Union. In another few days
your bank will discover that the check is no good and zap it out of your account.
In my opinion, the problem lies with the bank's policy of crediting your account
before the check actually clears. I believe what people should do is to deposit the
bad check, then withdraw the amount as cash. Then the bank will be left holding
the bag. You'll have to return the money, naturally, but if enough people do this then
maybe they would think about changing their policies.