Is zakat obligatory on money saved by a retiree to buy a car for work and to improve his living conditions, knowing that a hawl (lunar year) has passed on it, and that he does not possess any other money to pay zakat?

If you return the money to its owners before the end of the hawl (one lunar year), then there is no zakat due on you. However, if the money remains in your possession until a hawl passes over it, then the preponderant opinion among scholars —which is the view of the majority— is that there is no zakat due on you if the debts consume all the money after being deducted from the money that reached the nisab (minimum threshold).