Had to pay in for a same for same exchange

I bought jeans in week 1 of the Woolworths sale. I paid discounted price on tag as well as a further 10% off as I have a Woolworths card. The jeans are too small so I took them back yesterday - week 2 of sale. I found the exact same pair of jeans just a bigger size so it was a same for same swop. The customer services lady said I have to pay in the 10% discount I got in week 1 because it does not apply in week 2 so basically the sale goods are more expensive in week 2. I feel this is unfair as it was a same for same swop so why do I get penalized.. If the jeans had fitted in week 1 I would have paid less. Doesn't make sense. I wonder what CPA says about this