if you had paid 20% down payment + first year insurance 4%, processing charges, registration, all comes to 150,000. You say you have paid 31 installments @ 12000, this comes to 372000. Add this to 150,000 your total is 522000. You say 29 installments still to go. 29x12000=348000. add this to 522,000, its 870,000.
Other way 12000x60 installments =720,000. Remaining is insurance as markup is built-in in your installment.
So basically what you are saying is you need 550,000 from the buyer plus the liability of paying remaining 29 installments of Rs. 3480000. This 2003 Cultus will cost him around 9 lacs. How do you reconcile with it. Ok Lendi Renzo kit for 350000. But then Karachi registration poses some reservations at least in Islamabad.
My suggestion would be to pay off the lease to HBL and then explore the market.