Baby city bad to their customers

I bought a breast pump at baby city Loch Logan in waterfront only to find out that it is not working and bought the Medela one thinking that I will take the first 1 back when everything settle, I took it back they checked it out but the problem is I lost the till slip. I asked them to print it out because it still show the barcode and they refused but the manager said the pump shows that it has never been used before but they cannot take it without a slip. The lady at the till still remember me buying the breast pump but she cannot do anything since the manager insist we bring a slip. Please help me out because we paid lots of money for it and can use it to buy the baby something else that is working and what she is needing.