What is wrong with my kittens leg?

First off, your kitty is super cute. Sadly its kind of common for kittens to come with a lot of problems because the previous owners didn't care enough/ didn't have the money to take proper care of them. You should get her checked out by a vet asap. Hopefully the leg thing is just a little injury that can be easily fixed, but you need to make sure she doesn't have worms or any other parasites or diseases that come from poor living conditions. You most definitely can't take her back and the sooner you get her a clean bill of health the better.