Sales prices modification

Hi Everyone,

I would like to introduce my issue on AX 2009.

On my SO, I have defined  manually a sales price for the item 1 : 10€

I confirm the SO.

But when I invoice it I have a new price on the lines : 3€.

For information it's a intercompagny order and the new price is the purchase price of the item.

Could you help me ? 

Do you know how it's possible to change the price on the SO? 

Thanks