I suppose there's a technical legal analysis of this situation that leads to the fair result (e.g., dealer has a possessory security interest in your car title to secure payment of the $400, or some such similar nonsense). But who cares? Pay the man the $400 you owe him -- go in person with cash, and ask him to put the plates and the title on the counter before you hand it over. Expect to be scolded by the bankruptcy judge if you try to force the dealer to do anything without getting that $400.
I think the dealer is the one who will be scolded by the court. He conveyed title without retaining a lien, which should be his problem, not yours. Now he is withholding property of the estate, or at least impairing it. Not even a secured creditor can do that without obtaining relief from the automatic stay, and he isn't secured. He needs to turn over the title and plates.
From a bankruptcy rule perspective, if they have not provided the title due to you filing, this appears to be an automatic stay violation.
Why? They are appear to be collecting on a debt that is covered by the automatic stay. Even if they did have a lien, they could not collect on the debt.
This is not the end of the story.
If you enter into a credit relationship that closes to bankruptcy, it has the appearance of fraud. The dealer has the ability to sue you for fraud to prevent the discharge of the debt. If this happens and you lose, you would owe $400 and attorney fees.
How much risk? Not sure. Given they are not complying with the stay, the risk would be to of you being sued, furthermore, they will have to hire an attorney. Very expensive.
This assumes the security agreement was not perfected timely.
As a side note, this will take time and you may have to file motions and go after the dealer. Paying $400 can be the cheapest and the least amount of hassle. It is legal to pay the dealer.
Please contact me directly with document for a free 30 minute consultation to get more concrete advice. This is not legal advice. I don't have enough information to give actual legal advice. I can only take the limited information presented and provide a framework to know how your situation may turn out. I may have questions that bring up issues you did not think were important but make a big difference.
If the dealer still has your title, I believe that they do have a valid lien as a result of their possession of the title (at least in Virginia). Even though you did not sign anything, the dealer has a posessory lien against the title. Moreover, if the $400 was for the processing of the title and registration, you have an executory contract which you have now breached by not paying the $400 - how can you hold him to his duty to complete the title work for you without paying?
If he is unsecured then he has to give you the plates. Your bankruptcy attorney can file a Motion for Sanctions in your bankruptcy and you may need to report them to State licensing authorities.