Is Zakat obligatory on the remaining amount (90%) of the car's price that is in my possession but has not yet been delivered, knowing that I have only paid 10% of its value and the delivery date has not yet arrived?

If a hawl (a lunar year) passes over the money while it is in your possession, before you pay it to the car owner, zakat becomes obligatory on you because it is still under your ownership. You are considered indebted for the remaining price of the car, and debt may affect zakat by nullifying it or reducing it.