Unfair treatment

On the 24th of Dec '18, i returned a top at Truworths Brooklyn in pta for a factory fault that i bought in Nov during sale in exchange of another one as it was still available because I liked the top. The store manager traced the transaction on my account and seeing the fault she told me to top up the difference as the sale is over. I didn't why I should top up the difference as the fault was not on my side.