On returns, do stores have the right to keep some of the sales tax made on the purchase?

Item was bought at Kmart in Philadelphia at 8% tax rate. It needed to be returned the following day at a different Kmart location, which is in a 6% tax rate locality. The sales clerk agreed to refund the purchase price but would not refund all of the sales tax paid with the original purchase. Shouldn't the store have refunded all of the sales tax? If so, how do we go about getting the rest of the sales tax back?