Samsung Store Service and Voucher differences

I have upgraded my Vodacom contract and my new device is a Samsung S9 which come with a voucher. The new Samsung S9 includes a cover as well. I've decided then to rather use my voucher of R275 for another charger. Vodacom didn't have a charger in stock and referred me to the one Samsung shop in Menlyn. The store then said they don't have any stock either and referred me to the other Samsung shop in Menlyn when I got there they refused to take my voucher because their voucher value is R500 and mine is R275. I was then forced to go back to the Vodacom to take another cover. I just think it is pathetic.