If you have wrongful possession of somebody else's truck, you should give it back. If the truck was given to you as a gift to keep, then you can keep it. If you receive a Summons and Complaint, you usually have to file and serve an Answer.
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If someone is threatening to sue you, there is not much you can do to stop them if that is what they want to do.
Based on your description, it seems the only way to make sure you don't get sued is to give the other party what they want. Otherwise, you can wait and see if they sue, and then defend.
I don't know what the value of the truck is, but that might be something to consider. You could always try to settle the dispute with the other party, by offering to pay them something for the truck.
When faced with a threat of a lawsuit, it is always a good idea to count the cost. By that I mean it can be very costly, financially and otherwise, to be involved in a lawsuit. Defending against a lawsuit should involve the hiring of a competent attorney to defend you. The cost of a good attorney defending you through a trial could easily cost more than the value of a used truck. If you try to defend the case on your own, you will be like a fish out of water, and if the other side hires an attorney, you could end up being tricked, confused or overwhelmed by the process and procedure.
You need to consider the non-financial costs as well. Defending a lawsuit will take time away from you, and the emotions that are stirred in the middle of a lawsuit are powerful. The anger, fear, uncertainty and anxiety of litigation have caused many people to suffer physical symptom including depression, mood swings, headaches, back aches and a panoply of other issues.
So, you need to count the cost now. Settlement is superior to litigation if it can be achieved. See if there is a middle ground that can be reached. If you cannot do it yourself, perhaps you can have a mutual friend, or a neutral mediator attempt to resolve the matter before it gets to a lawsuit.
I wish you the best of luck.
Pedersen Heck McQueen, APLC is an Irvine, California litigation firm assisting litigants in all Southern California counties.
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