Voucher *******

I was sent a R100 birthday voucher on Monday 11/01. As I needed some clothes I decided to go to Edgars today and purchase a few items. I found some trousers on promotion and a shirt with a 50% price reduction. As I paid I informed the cashier that I had a R100 Voucher. I was told I cannot use it on any promotion otherwise ! would have to pay full price. What the good is a voucher that you cannot use??