Disappointed in Clicks for inconsistencies

I bought a 3for2 on a shampoo product on the 18th tg of December. I wanted to do an exchange as I bought the incorrect product. I was advised by Customer Services that the promotion will still be app**** e en though it has ended because I had purchased it when the promotion was running and my slip was evidence of this. To my dismay I was misinformed at Clicks Mall of the South in Johannesburg and I was told only to take 2 products with the exchange as the promotion has ended. The supervisor on duty kept my slip but I have the exchange slip with me. There are obvious inconsistencies in staff applying policies correctly. What can be done to rectify this?