Hi Jamshed
At the time of order confirmation system usually uses planned activity prices (KP26) for postings. But it depends on your valuation variant assigned to your order types (OPN2).
I doubt that at opn2 for your val variant under 'Activity type/process tab, you have defined 'Actual activity price' at first priority hence system is demanding you the actual price at this time.
Else at the time of period end activity we have to calculate actual activity prices (KSII) and revaluate the orders.
Regards
Rajneesh