We are trying to use the substitute items  BM-J to temporarily substitute an item for one that we cannot get for a few months.  We entered it as a global substitution.  But when we issue the item that we don't have to a work order it does not do anything but give us a warning that we will go negative- no mention of substituting the other item.
Is there another setting that we have to change to use this?  If not, what exactly does entering approved substitutes accomplish?