Returning as son got a gift card but wouldn't let me re-buy

Was my sons birthday so we took him to the local smyths toy store colchester and bought him some lego took him home and someone had got him a gift card. He didn't want anything else so I decided to take the lego back and get a refund so I could re-purchase the item when I told the person he told me I couldn't. He's all ready had the money for the gift card so I don't know what his problem was he was either the manager or a supervisor this was the worst bit of customer service iv ever experienced and iv been in retail for 22 years I will only go in to the store to spend the gift card then from now on will go to toys r us.