Bad customer service and not training on the management

I am so angry, Ive just came out to one of the Built it store to buy something for my washing machine. I bought this thing and I was not sure that I bought the right. I told the associate what I wanted, and he gave it to me. So told home let me buy it and will open it here since we are not sure. Then if it's not a right thing will exchange and take something else. I don't want cash. Bare bin mind I'm sti inside the store. Didn't even went out. The supervisor I don't know where she got her training form. Her name is Boniswa. Told me that she can't take it because I have open the pakckge. They can't resell it. I mean I'm also a manager. I know thses things. If I'm not certified with the product I return it. And if I don't have a slip I can't take cash... I don't how they promote thses people or the company trains them. It's just bad. I make sure at my store no one leaves not certified. And she was even proud to tell me her name as if she's doing something right. And I didn't even damage the product. I was there with them and nothing in their policy says about the packaging. I've read it.