I'm guessing he cashed it and you're now looking at the image of the check online through your bank. As Zerin pointed out, I do believe his cashing the check is acceptance of your offer to pay $600 and consider the "bill paid in full" though, his messing with the check is very sketch.

If you can't bring a contractor to court, get some signed paperwork from a couple different shops that support your claim that 17 hours is ridiculous. Formal estimates for the work. If he can't prove his hours, ask the judge to charge the job based on the estimates you provided. That seems like the fair solution here.