I don't know what Pottery Barn's policy is, but most stores won't just give you cash back these days. Generally, it's a gift card. You might want to check their website. What it does actually give you is, you can get credited for the exact amount that was paid for it. And if the purchaser brings it back with the receipt, usually the computer can just credit the credit card it was purchased with (at least in Target).
It used to be that stores would take anything back this week--not sure if that's still true.