Purch line amount is adding Fixed asset book acquisition price

Hello All,

I have scenario like, example I have one fixed asset with one book which has acquisition price of 1000…

Now I have created purchase order with 2 lines nd these to lines associated with above fixed asset and they have total amount of 500…

When I confirm, and invoiced the purchase order , the total amount of lines is adding to Fixed asset -->book -->Acquisition price

I have to do some customizations at the point of adding the amount to book acquisition price …
Can anybody help which class or method I need to look for that or exactly what is the method that is updating the acquisition price…

I have debugged the scenario but could not find the right place