Decathlon Gave the Wrong Size of Inflatable Mattress As Replacement

I bought this 140 cm camping inflatable mattress from Decathlon and it was pretty good at first. - the mattress started deflating all by itself. Pretty weird I thought, also kinda worried...so took the mattress back to the Taksim Decathlon store where I got it from. the folks there checked out the mattress and, after giving it a good look, said it was cool to exchange it. Unfortunately, the replacement mattress they gave me was smaller than the one I bought. Honestly, I hadn't really paid much attention to the size specs before. first didn't notice it but then when I started using it realized that the mattress was way smaller, like, it was basically useless for what it was supposed to do. I quickly reached out to the Taksim Decathlon store to let them know I wasn't happy and to explain the size mix-up. even though it was obvious that their system was messed up, the store people were all like, "Nah, we can't take the mattress back because it's been used and has a little stain." If they admit that they messed up, why isn't the company owning up to it and fixing it? Basically, I just want to make sure this situation gets sorted out and I get the product I wanted.