If you want to sell your share of deferred money by the government in return for a car or anything else, this is impermissible, because selling the debt on the part of whoever does not (Part No. 13; Page No. 256) own it is impermissible whether in return for a car or any other commodity, and as such, this kind of transaction is legally invalid.However, buying a car or another good on credit then selling it for cash to fulfill your need, this is called
Tawarruq sale (selling by installments, then the purchaser selling the product to a third party for cash) and it is a permissible transaction provided that the purchaser does not sell it to the creditor from whom he bought it in the first time.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.