VAT treatment for trade-in product

Dear friends, I would like to understand VAT treatment in a scenario wherein the customers are offered to exchange their old products for the brand-new product. In this case, customers are required to pay only the variable consideration resulting in a discount on the price of the new product. So, the old product returned by the customers to the retailer is adjusted in the price of the new product - is the consideration by the retailer for this old product liable to Vat as a part of the total selling price of the new product. Any views will be helpful. Thank you