Credit refusal

I bought 3 pairs of shoes at pnp clothing blue route because I wasn't sure of my daughter's size. I chose one and wanted to return the other two but didn't have a slip. When I got there I saw a top and thought it would look nice on me so I exchanged. When I got home it didn't fit. So I went back to credit the too small top and was told that I cannot get a credit only exchange. I explained that I didn't have time and that I don't want money back just a credit to come back when I had more time. The store refused said it wasn't there policy.

I understand that without a slip I cannot get my money back but fail to see how I can't get a credit note.