My uncle is a carpet installer, and does a lot of his work through Lowe's, but they're really weird about what happens with any of the extra or returned carpet. Obviously, contractors are encouraged to offer the unused portions to the buyer, which they often accept. But I've worked with him on some jobs where the carpet was either not what the person ordered, or the person did a lousy job envisioning what the carpet sample would look like as an actual carpet and they find some nonexistent flaw with it. At that point, Lowe's will actually take back the carpet, replace it with something the customer does want, and makes the installer return the carpet to them. They don't reuse it, though, and it's my understanding that even though they are insistent that the installer return the carpet, it's eventually just discarded.
TL;DR Lowe's is like the H&M of home improvement.