So, for your requirement set the same price for your condition type before the actual price has been defined.
After the actual price has been defined, modify the actual price directly on the purchase document. At that time, the purchase document keeps different price for both of each the Actual and Temporary condition Master(Here, Condition type also can be modifiable).
After the modification of the price, the credit memo that is the amount of the volume multiplied by the difference of the price has been created for each target document.
Please try this and update me.
Anandkumar KS