Is it permissible to value an old car against the purchase of a new car from the same dealership, and is this considered to be among the category of "two sales in one sale"?

The scenario mentioned in the question is permissible and does not fall under the prohibition of "two sales in one sale." The Permanent Committee for Issuing Fatwas has permitted the exchange of an old car for a new one with the payment of a difference, considering it a sale of one car for another with a difference in value, and not usury (riba). Scholars have interpreted "two sales in one sale" in several ways, including bay' al-inah (sale with repurchase agreement), stipulating another contract within the sale, or setting two prices for the commodity (cash and installment) if they do not agree on a specific price upon parting. In all these interpretations, the scenario in question remains permissible.