Pathetic service ever.

I bought sweat pants and a top for my son at H&M canal walk, the pants fit him properly however the top is very small. 4 weeks later i returned the top to exchange the size the manager told me he can't assist as i did not have the purchase receipt with me however the original tags were on the item. i understand they have a policy in place but all i wanted was to just get a size up they refused and what made it worse is that the items is still available on the H&M floors with the same amount.