reg scrap in subcontracting

hi,

i am working on subcontracting scenorio, i created material x as scrap and i define in -ve quantity in Bom . once i receive the finished product from the vendor the scrap doesnt come into picture .some of the threads show that scrap come automatically with 545+o movemnt type. how it happens? explain where i am missing.

regards

muthuraman.d