Diapers that don't fit?

Lindsay • Atlanta
We couldn't find the diapers we had been using and got a different brand of NB size. 
They say up to 10lbs and our lil boy is only 8lbs so I know it's not the size. But we noticed they squeeze his legs so we stopped using them. 
Are we able to return them even though they are opened and we used a few? It was a $40 box!
What do you do?