bad service

Me and my partner went markhams at sandton we puch****d a top that had a R100 off mark but when we went to the counter we paid full price for it and and was told that the R100 mark down is not available anymore,what I fail to understand is if the R100 mark down was not available anymore why was the R100 mark down adverti*****t there?why didn't they overide that and let us pay with the mark down. I'm very unhappy about this and I need my partners account to be credited with the R100 and please tell your staff to stop advertising falsely