Misleading promotion card

The Spar Group has a promotion running at the moment where you have to collect stickers and complete a card, to buy glasses at a discounted price. When I wanted to complete my card with the last to stickers on my card, the cashier at Tops in Sabie told me all my other stickers on the card are not valid as my original till slips are not stapled to the card. No where on the card does is state that all original till slips should be stapled to card in order to qualify for the discounted price on the glasses. I spoke to the manager, Elliot, and explained him that I need to submit my original till slips for refunds at my work and asked him where does is say that original till slips need to be attached to card. He then told me that it is not stated on the card, but on a mail from the Spar Group to the various retail managers. He also said as I am a local from Sabie, he will make a exception and still give me the glasses without my till slips being attached. I do not feel this is acceptable as they cannot change the rules and regulations without mentioning anything about this on the original card and not once have the cashiers told me that original till slips need to be attached for the card to be eligible. I already had one completed card and another only short of two stickers.