Problem while assigning Differant valuation class to same material

Hi,

I have one finished production called - 1000

Now i have assigned this 1000 - finished product to valuation calss 7920 and plant 1100. The same material we use for plant 1200 also.. Now i am trying to map the same 1000-finished product to plant 1200 by creating 7921 valuation calss.

But the system is not allowing me to give valuation class 7921 for material 1000 since it was alread created with 7920.

I am a FICO consultant and dont have much idea. please help me how to assign same material -1000 to differant plants with differant valuation classes?

Thanks

Anil