Wont exchange item

I made a cash purchase at Miladys Cresta like I usually do, to the value of R1500-00. I was away on business and two weeks later went in to return one item that was not ideal. I had the blouse with the tags on, and the payment receipt but cannot find the actual transaction slip. I was told I can return it after some reluctance but at the price on the system which is R100-00 less that what I paid. I cannot understand why I am being asked to exchange at a lower rate when the price tag shows the original. I was told that nothing can be done. I find this unacceptable. If I go to a builders warehouse, they can find the transaction albeit cash based on the card receipt!