Is it permissible to increase the number of cars in exchange for delaying their payment?

It is not permissible for the buyer to promise an increase in the deferred price in exchange for his delay in payment, because it is usury (riba), which is what the people of Jahiliyyah (pre-Islamic era) used to do. The obligation is to grant respite to the one in financial hardship.

However, if the seller grants him respite without an increase, and then the buyer voluntarily adds an amount, there is no harm in that.