Is a traveler obligated to return the money he took from his friend to buy a car after it was lost and shipped to an unknown location, knowing that he was not a car dealer and they had agreed on a subsequent fee?

The agent is not liable for what perishes in his possession unless he is negligent, because he is an أمين (trustee). So if he was negligent in dispatching the car, he is liable for what results from that. As for the allocated sum, if it was a wage that the agent earned for carrying out the purchase and shipping, then it is due to him. But if it was a fee for delivering the car, then he is not entitled to it until the car arrives.