Refusal to supply

Today I went to Clicks in the Pavillion and saw that the GNC lean shake was on promotion. I saw the tub in which it came has discolored from white to a yellow colour. When I looked at the expiry date and noticed that all the tubs expired in May 2018. When I asked the store assistant if I may please have a tub that did not expire he said that then Ill have to pay full price. The promotion does not stipulate that it is only in respect of products expiring in May 2018. Therefore the refusal to supply is unreasonable.