I was hired to drive a truck with lots of stuff upstate. I hired a friend to help me. The people who hired me knew about this friend. At one point he took the truck with empty suitcases in the back. He forgot to close the door in the back. One suitcase fell out and could not be found. Do I pay worker? Who is responsible to pay owner of suitcase for his loss? If I paid worker already do I have the right to ask for money back?

It is difficult to give an answer without talking to all parties personally, to find out all the details accurately. Therefore the following answer is according to the description in the question, but in practice one should consult a rabbi in person. You and your friend have the Halachic status of a paid bailee (Shomer Sachar) which is responsible for theft, loss, and needless to say also negligence (Shach, Choshen Mishpat, 303, 1). The case you describe is a classic case of negligence by the fact that he forgot to close the back door, causing loss of one of the suitcases. This case is considered negligence and therefore he must pay for it (Shach, Choshen Mishpat, 291, 3, and all the more so by a paid bailee). The person who employed you can demand it from you, because you're the only one who was employed by him and must bear full responsibility. Therefore you are responsible to pay the owner for the suitcase. You can of course demand it from your employee who must pay for the loss. You can hold back from paying him up to the amount of the value of the briefcase and demand money back if you have already paid him. It is important to note that the amount to be paid for the suitcase is not the price of a new one, but the price of that particular suitcase. If it's used or worn, the price drops accordingly.