Shoppers Drug Mart review: sale product

Hi, I went to Shoppers at 25 Great Lakes Dr, Brampton, ON L6R 2S5 on Wednesday July 10th. There was a product which was on sale for 3.99 and it had a 4$ reduced sign on it. I wanted to buy but the cashier said that I cant do that. He asked the manager to come and he said that the 4$ reduced coupon (discount) will be applied to the original price (6.99) not the sale price. I told them it is on sale but they told me they can not apply the coupon on the sale price as the sale price was lower than the coupon amount. This was their mistake but I ad to pay instead and I decided not to buy that. I would appreciate if you clarify to me why the people at shoppers are not trained to resolve these situations.