false advertising

Hi there, i went to Spar in Thabong(sebokeng) to buy milk on Tuesday the 03/03/2020 and found one that expires by the 08/03/2020, however on the 06/03/2020 the milk went sour went back to the store to and exchange for the milk cause it went sour pior to the expiry date and was told (Superviser Anastsia) that when I opened the milk it was fine, and I did say yes however why did it go sour before the expiry date they had no answer to that, I was to come back around 09:00 am till the manager gets there which I did and then Mr Pienaar (MANAGER) said that I used the milk and its less that than the 2l i bought so they cant refund or exchange for me, I even told him that i don't mind getting footage from the company i work for to see how the milk was taken care for, according to them i was not suppose to have used the milk till it expired