$720 would buy you a brand new bumper from the dealer with a markup and have it painted by a body shop. I find it odd that they would charge over 10 hours to fix a minor blemish like that.
Unfortunately regardless of if you did it or not, you are at fault since you can't prove that you didn't do it. You could refuse to pay, but I would assume that would mean you can't work at that golf course anymore. I would tell the guy that you will have it fixed, but you will get an estimate yourself. Just my .02
This really goes to show that you should do a walk around with the customer before you start working on their vehicles, and then have them sign some paperwork.
On an unrelated note, if you aren't making enough to pay yourself you either have too many employees, are paying them too much, or not charging enough for your services. Sounds like you might need to step back and take a look at all of that. No reason to work that hard and not have something to show for it.