Decline an exchange that is clearly an error on your side.

I bought 2 pairsnof boots in February. In Apri during the lock down, we noticed that the one pair had two different sizes in the box. One 8 and another 6. The pair was given to my mom as a present and she notices this fact. I contacted them but couldn't lodge an exchange as they advised that I can only so this when the lockdown is lifted. I waited till now and they are declining to exchange the boots. What must I do with boots of different sizes that they delivered to me? Clearly there is a set storage that I has different sizes in. Why decline an exchange of goods?