If you paid by credit or debit card take your staement in, they can then see proof of purchase if they want to, but I would ring the store first, ask toi speak to the manager and explain what has happened and arrange to call when they are in to get it removed, what sort of tag is it? if it is a dye one put the jumper in the freezer in a pastic bag and when the dye is frozen you cabn cut it off with metal cutters, if it is a magnetic one then you need to but a magnet, a strong one.