Despite the request being made to Seniors I'm hereby attempting to suggest something here. Hopefully Senior members here will be here guiding us for next course of action. Meanwhile here is what I can suggest.
First of all, just take a deap breath and chillax. What you have done is quite normal. Let me tell you what would have actually happened. Due to defaults, company was quite in desperate mode to make good the defaults made by the clients. So in such situations, the first target is the person who sealed it. Definitely top brasses won't be facing the axes over this and first victim is sales or deal-clincher. In your case it was you. So they asked you to return back the incentive or stay back so that they can recover somehow that money. Then over the period they found a website etc.and blamed you for the dual employment.
Well I have 'n' number of ways to go around it but being a public forum I certainly can't suggest out here.
They have fired you and I presume you have already exited that company amicably or would be exiting very shortly.
Your actual concern seems to be "If they can file a case against you?" Lets keep certain things clear here. Cases are not filed just like that. There's due process of law. As per what has been told here, your company is already in bad shape with debtors defaulting and thus should not be capable to take on another law suit. Frankly speaking I am expecting not more than 10% chances (unless facts changes). At max if they feel like of anything, they will simply be serving a legal notice. Just contact some good lawyer for reverting on it.
At last, once again I would say, simply chillax. It happens almost on daily basis. You were made scapegoat of it and are being made to feel guilty unnecessarily. As long as you did no wrong, there's nothing this Employer can do to you.
6th October 2013 From India, Gurgaon