Ulf,
That 1/2-inch will make a difference in the way the tire sidewalls sit and therfore how the tread remains applied to the road. On the front, it would make a difference in how your car responds in the event of braking in a turn. It might make a difference in how the tire responds in a panic stop. In the US, shipping a rim by UPS ground is not very expensive. I would call the supplier and see if they can swap it out for you.