Hi Amr,
When you enter the item for the first time, system will calculate the pricing depending on the SUoM entered by you. Now, when you change the sales unit, system will not automatically re-calculate. When you go to Condition (pricing) tab of item, there is a button Update, this you can use to trigger recalculation of price. Click on this button and system will ask for updating method, select 'Recalculate Pricing' (I guess B is the option, you can read the description in a dialog box and choose appropriate), it will update the price according to new unit.
Let me know if you need any other information.
Regards,
Amit