Unable to Get Correct Size Replacement for Mango Dress

I purchased a Mango dress via Trendyol on 2 December with order number 9998866385 during the November sales period. I selected size medium, but I received a small size instead. I believe this was related to the discounted sale period, but the product I received does not match the size I ordered. I contacted customer service and requested that they change the dress to the correct size, medium. They told me that the product in medium size is out of stock. However, when I check, I can clearly see that the medium size is still available. I kindly request that the dress be replaced with the correct size, medium, as originally ordered, and that this process be completed without any additional cost or loss of my discount rights.