Sounds to me like you have set it up incorrectly or you are not understanding the message. Create the bom with one line, the negative line, then post the consumption journal, I bet you are not told the item is not on inventory then.
U wont do this way, just keep this Simple Bom for A will have B , Bom for B will have C and D. Being B as ur sub-assembly does not mean u will keep negative quantity.
It is just a multilevel BOM. So first u will produce B and consume C and D for that and will then produce A and will consume B only.
Remove C and D from your BOM, then load a new works order and post the consumption journal - what does it do? I am presuming it is a released order and not firm planned as you describe.
Seriously which part of my statement "Remove C and D from your BOM, then load a new works order and post the consumption journal - what does it do? I am presuming it is a released order and not firm planned as you describe. " did you not understand? I am trying to prove it is not the negative line but your understanding that is the issue.