In App purchase not working

I have bought the Future Furniture pack inside the Apps Marketplace.

However, I am not able to use it as it won't "install" within the app. It always says "this should not take long" after the AppStore told me that I have already purchased it.

I am billed twice for that! 6,99 EUR and I cannot use it at all!

Please tell me how to proceed as this is absolutely not acceptable!

Thanks.