Dear Vijesh
no need to replace the old item substitute and there should not be any warning message also
A bit confused but as per my understanding you want to Give alternative material when stock is not there ..if it is right please fallow below steps ...
While creating Condition record in VB11 T.code ...Give Main item (A) 100
then press F2 ..Here give Alternative material like B
Now while creating sales order first system will take old material i mean A
But when stock is out of the plant automatically B Will come into picture
if it is not your requirement plz let me know
Thanks
venu 