Supervisor not honouring pnp policy

Whilst buying a product, the displayed prices indicated R150 for 2x six pack. I took 4 x six pack expecting to pay R300. Cashier scanned all 4 and bill was R315. I queried and she said it’s not on special I then paid in order to not keep up the line and went back inside store to get the promotion pamphlet. I brought it back and the supervisor SIPHOKHAZ was called. The cashier explained and informed that double the difference should be paid. The supervisor laughed and said it was cashier fault and she will NOT be honouring the policy and paying out double the difference, she had a laugh and smirk on her face as if I was wrong for saying the policy. I walked out as I was appalled that a supervisor would act this way. This was at Pnp rosmead liquor. I have all the required photos to prove my claim.