Unhappy with customer service

My mother bought shoes at the Bochum store only to get that it's different sizes 5 and 6. She noticed that when she got home but she had lost the slip and she went back to the store to ask what can be done, she was told to bring the shoes back. When she went back with the shoes she was told that there's nothing they could do since she lost the slip and she bought them cash only had she bought them using an account she was going to be assisted . Question is who's negligence was it to put two different sizes in one box? Does it mean only customers with accounts deserves a special treatment?