Is it permissible for the debtor to pay the price of the car in cash and bear the difference as interest to the showroom owner for the money he possesses?

The hawala (transfer of debt) is permissible. The intention is for the buyer to transfer the remaining price of the car to the showroom owner from a debt owed to the buyer by his own debtor, so that this debtor pays the showroom owner his due from the debt he owes. This is based on the Prophet Muhammad's (peace be upon him) saying: "The delay of a rich person (in paying debt) is oppression. And whoever is referred to a wealthy person, let him follow (the referral)."