Is it possible to sue a lawfirm who made a bogus invoice just to keep your retainer??

It's always possible to sue anyone for anything. Realistically, though, to hire a lawyer, you need to have a strong case. Here, my first question to you would be to see the prior retainers. If the retainer balance had been carried for a period of time and then a spate of activity occurred after you had made an appointment to come in to recover your unused retainer, you're off to a good start. The next question would be whether you can document that the attorney had prior knowledge that you were coming in for the refund.

To prevail in court, you would have to prove that the attorney knew you were coming in for the retainer and purposely created charges for work not done or, in the alternative, created work that he knew would be fruitless, since he had knowledge that you were going to fire him.

If your intent at the time of making the appointment was to fire the attorney, you should have first asked for your latest statement, then fired the attorney, and only then made an appointment to pick up your unspent retainer and your files.