Disappointed regular customer

Incident occurred on Sunday 14 June. My sister needed to return a pair of black ankle boots (R599) I bought for her birthday on 3 June 2015. Since then, the boots went on sale and unfortunately I had misplaced the receipt (I am fully aware that customers are advised to keep these but Cotton On customers are human too). I am also aware that retailers have to keep record of all transactions for legal purposes so I asked the manager to print the relevant receipt (I gave her the exact time and date of purchase). The manager (no name tag) was flat out rude saying that this would be too much effort and she would only refund at the sale price. About ten minutes of arguing and a threat to call head office persuaded her to print the relevant receipt with upmost displeasure and appalling attitude. The boots were exchanged for items more expensive, so actually a win for Cotton On. Totally unreasonable bad customer service and from someone who wishes to continue shopping at Cotton On, I would appreciate some recourse.