it is preferable to invest the installments of those people who entrusted you with their deposits for their benefit, and you are rewarded for your effort to invest money for those who do not know trade transactions. However, if they allowed you to
benefit from the installments until the end of the credit, there is nothing wrong with that.
the owners should pay the due Zakah on the entire sums of cars sold on a yearly basis or they may entrust you with paying Zakah on their behalf. Thus, you should give it to those who deserve it. You will be rewarded as long as you do a good job and have a good intention.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.