Handling Scrap in SAP B1

i have 20% scrp generated on bom

but the fg gets the complete price from the rm

and scrap gets over the cost as defined in price list

if the rm input is say 100/- and fg is receipted at 100 and scrap at say 20

thr is a variance of 20

i need the fg to get 80 and balance 20 to scrap

How is it possible in standard SBO?