Credit note discrepincy and unsatisfied client at Midas

I bought brakes for my Corsa on Sunday. Discover that it does not fit, took them back on Monday 24th which the guy at the counter told my sun in law that he need to clean it and bring it back. I took the brake shoes back on Thursday, the guy at the counter told me they cant refund me as I need to go and clean them first. On his advise I bought a can carborator cleaner and sandpaper. I cleaned the parts and took them back on Friday, 28 June. After entering I gave the parts and slip to the guy at the counyer, asking them to refund me now as the parts was not the right ones for the corsa. He took it to the lady sitting at a desk and was busy on the phone. She told him that he must give me a credit note as they are not going to refund me. Took the parts and sigh on the slip. I was furios of the way they handel their clients. My point is why didnt they told me that from the start, before they suggest I buy cleaner and sandpaper and letting me come back by telling me another story. Now I had to buy something from them in order for me not to lose my money via a signature on my original till slip? Wasnt they suppose to write out an official credit note before I left their premises???