Game price beat

Hi. Today I went to game Balito to phurcase future life cereal. Game had a box for R39.99 while woolworths had it for R24.99. I have a large family and we all consume this cereal for breakfast daily. I generally purchase 20 boxes a month. When I got to the till the cashier was going to put the sale through but the store manager stopped her and advised I cannot phurcase 20 boxes as i asked for a price beat due to woolworths having it at R24. 99. He only wanted to give me 6 boxes but then the store manager said he will give me 8 boxes. I am totally dissatisfied by the service I recieved as I have been a loyal supporter of game all my life. They guarantee to beat any price if you find it lower at another store but suddenly the manager says I am buying bulk. How can 20 boxes of cereal be bulk when that's a months supply for my family. The cashier had no problem in putting the sale through but the manager stopped the entrie process. I would appreciate feedback on this and please can this matter be addressed