Disappointed

I'm struggling to understand why Woolworths wants to keep my R45, i bought a kid's dress few weeks back, payed R50 extra from the shopping voucher. The dress is small, price tag still on, I don't have the till slip anymore. I get there to exchange the R150 item for a R300 item. But to my shock the dress has been further marked down to R105 so thats all the can pay me out for. I offered to bring my bank statement cause I just can't let my money go like that but NO!!!! that will still not work. Like really you plan to keep my R45 for no ethical reasons!!!!!!!