So, go to the hearing, and tell your story like you told it here. Be sure to have as much documentation as possible, including sworn statements from the employee (or better yet, bring him along). I'm not a lawyer or anything, but a couple things that I suspect may work in your favor:
1. It's not your fault that they didn't charge your CC like they were supposed to. If they don't take their payment, I don't believe you're under any legal obligation to go out of your way to provide it.
2. An employee forging your signature may just nullify the whole thing.
3. The employee's statement to your arranged payment will be crucial - you were told by an authorized representative that you could take your time in paying, and that it would be $600, so it's not like you were trying to skip on the payment.
4. You DID go to the company in good faith to make your payment - it's not your fault that they weren't there anymore (and I assume, left nothing behind to indicate how to contact anyone?).
5. Did they close down, or just move? You owe the $600 to the *company*, not its owner - if they ceased to exist, it may be determined that you owe the "owner" nothing.
So yeah, document everything, and go to the hearing... and keep us posted! If you end up getting to keep the $600, you can take us all for drinks