Overcharged on product that's on special and store refused to give me double the difference back as states in policy

On the 26th Oct I bought a few items & only discovered when i was home I was overcharged on a product that was on special. Walked back & the lady who assisted me said wait whilst they check on price & item. She came back & said I must wait. She came back with what I presume is the difference in what I was overcharged. I asked her about the double the difference policy she took me to the store manager Maxmillian Chenjeras. I asked him the same question & he said & I quote " No no you see it's not for items on special" & he can't give to me. But he will sort something else out. I asked if he could put it on paper he refused. My stupidity was to accept he " promised to sort something out" I went back the next day (27th Oct) he hesitantly said he could do anything yet? Not wanting to be a "difficult customer" I said ok I will come again. I went back again on the 28th Oct where he reluctantly & with no remorse just said there nothing he can do. Hence why I had to put it on here. As they owe me the double the difference on a product i was overcharged. I shop there 2-4 times a week only because it's nearest to where i stay. I'm disgusted really as being an ex pick n pay staff member I'm taken back with the way he actually had no remorse. He blatantly refused. Honestly pick n pay Maitland is 1 of the worst stores I've seen. The staff has no etiquette. On cell phones when helping staff. Or chatting away with colleagues while helping you sometimes scanning items twice because of it. They hardly greet or ask you your smart shopper card. The store is filthy. As for the store manager who went back on his verbal word is a real ***** . Highly disappointed.