It sounds like you've already done it: you learned a lesson, and only lost five to seven hours pay. Bargain.
Please don't do this ever. There is no such thing as hitting them morally. If they are not paying you for extra 6 hours, they don't care about morality anyway. Value your work and get paid.
If yes, then just take whatever they give you and then don't bargain for that 6-7 hours. If they think you are easily manipulable, you might get more work from them. Take the work and you could be in a better bargaining position.