Leslie went to Pick N Go Supermarket to buy some groceries. He saw a poster in the supermarket that read "Buy 3 packs of Oceania tissue paper and get 1 pack free". Leslie took 4 packs of Oceania tissue paper and went to the payment counter. The cashier informed him that the promotion had already ended and that he would have to pay for all 4 packets of the Oceania tissue paper. Leslie then decided to not buy the tissue papers, but the cashier insisted that he must pay for all the 4 packs of the Oceania tissue paper.

Advise Leslie.