Try what my friend did recently in Bauhaas.
The staff could not exchange as they did not have the same model in stock so he demanded a refund.  Was told that this was no longer their policy since August, which happened to coincide with the date the product was bought - very convenient.  He was a offered a credit note instead for other products.  He told them that was not acceptable and demanded his money again.  This was promptly refused once more and stayed that way until he demanded to see the store manager immediately.  Surprise, surprise they were suddenly able to refund the money.  They obviously had a set procedure to adhere to until they realised that they were not going to get away with it and bringing out the store manager was obviously going to get their knuckles rapped.