What you have is a mess. I agree with you that it sounds like there was no verbal agreement, here. I do not see any basis at all for them to sue you for $1,200, when at the most, your "share" of this expense would have been $600.
What caused the impound fee to be so high in the first place? The plaintiff has a duty to mitigate his damages, and arguably, made no effort to do this. He cannot simply let his car sit in the impound lot for a year and say you owe him $2,400.
Having said this, once it goes to court, you never know what a judge is going to decide. It is possible that he might agree there was some obligation on your part. The other side of that is that you cannot pay what you cannot pay. They can sue you for a million dollars if they want. Even if they win, if they are unable to collect, they are out of luck.
Your summary does not state whether you are being sued in small claims court or district court. My guess is that it is small claims, in which case, there are no lawyers involved. You need to show up to court, or they will grant a default judgment against you. I would tell your side of the story and the judge will side with whichever party he believes. If you lose, I would try to arrange some kind of payment plan you can handle.
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I agree with Atty. Frederick that there is no basis for a contract (no bargained-for exchange), however, there may be another claim that could be successful (the car wouldn't have been impounded but for the marijuana possession).
Good luck! Please let me know if you have any further questions.