Cash back promise not honoured

Membership sign up promised R500.00 cash back on your first month if you present them with a voucher in a specific magazine on the day of sign up. It has been months and a lot of nagging from me and empty promises from the sales manager still achieved nothing. R500.00 is not small change and extra money was spent on buying the magazine in the first place. This is totally ********* and I consider it *****ing.