Prices on the board should be correct

Sunday Morning 13.08.2015 my son ask me to buy Ice cream i went to the store to buy the board shows R3.90. then i gave my son R4.00 but my son came back to saying money is short i didn't like it because and person who was assisting was like sorry prizes did change yesterday. i did inform them that according to consumer rights you pay the price that you see and he was like can i call the next customer. i know my rights when i visit any store and please educate the team members to know customer rights.