Clothing Disaster

Good Day, A week ago I purchased a pair of pants for Hayfields Branch. They were fa bit tight so went back to the Midlands Mall. Unfortunately I couldn't find my tillslip. The lady said no problem and put the pants by the till. I then find a bigger size to be informed that those pants are now on sale and that she has adjusted the price on the price tag. I paid R179.99 and she then marked them down to R139.99. This is totally disgraceful and unacceptable. Why do I loose R40-00 because your clothes sizes are tight fitting. I normally take a 36 but from precious experience I always take a size bigger yet they were still tight. At no time did she tell me that unless I produce the slip she cannot give me the full price. She marked them down without my consent. I now sit with a pair of pants marked down. Should I find the slip they still won;t change them. Totally unacceptable, 50% of my clothes are purchased form PnP but I won;t purchase clothes from your store any longer