Unfair Price Disparity Demanded by Vivense for Sofa Replacement

I purchased a sofa from the Vivense store in Çanakkale for ₺12,000, but due to incorrect installation by the manufacturer, we had to replace it. Despite the lengthy assembly and delivery times, the price of the seat skyrocketed. Now, the same model I bought is priced at ₺20,000. To avoid a recurrence of the assembly issue, I opted for a different model at the same price. However, Vivense is insisting on an ₺8,000 price difference for the replacement, even though both models were originally priced equally. I demand an exchange without additional charges, as both the returned and replacement products are valued at ₺20,000.