Doing the right thing...
In the freelance world, taking a job on means keeping your word. Once you sign up and agree to take that job, bailing out without negotiation can only end up working out badly for you. If you are booked for a job and something else comes up, you need to speak with someone; don't send an email or message declaring you're suddenly not available. Ask if they are able to find another crew member to do that job and if they get back to you with a yes then you've done the right thing. If they can't get someone to fill your place; remember that you agreed already to do the job; and by then fulfilling your agreed role you've done the right thing.
Do not expect them to employ you again if you bail on them.
From an employers perspective, once an employee bails they become unreliable and who says the next time that employee is booked on a job what stops them from doing the same thing and leaving you with a problem. There is also nothing worse than finding an employee bail on you and discovering that they've taken another job that runs across yours. That shows a complete lack of respect for you and the company you represent.
Always discuss the issue by speaking with someone.
Never, ever bail on your employer.
You won't be getting more work from them.