I am planning to let my boss know about it. Right now.
And yes I was joking about the million.
The thing is that I'm an in-house designer for a non-design orientated company so it's not like I am stealing business from the company. The opportunity came up because some other guy doesn't have a clue about making a proper logo. At least that's how I see it. But I don't want problems for myself which is why I am being straight up with my boss. If he has problems with it I won't go near it and won't be bitter.