Hello ,
you need to activate the External amount field in the movement type as suggested by Jürgen , you need to enter the total amount of stock while doing scrapping so that it will update the price you want.
but this will also affect your total stock MAP.
The scrapping movement wont be updated in the history as you actually cant scrap material with reference to PO. but if you show the material as return to vendor then that entry will be considered in the history.
Entering Material for Scrapping - Inventory Management and Physical Inventory (MM-IM) - SAP Library
you can try somehting like this
1) while scrapping use any available text field to enter PO number(you can make it mandatory for MT)
2) create a report based on above data to track the scrap against POs.
only issue in this i see is you need to enter individual po scrap and cant make a collective scrap for same material
Thanks
kunal