Dibyendu,
There is no date to be maintained in SAP system, and this is the reason why the system doesn't check the quantity against a period. I know and I mention that. The e.g. quoted is the business requirement.
Maybe the solution is to set 06 216 as W and to do a development in order to see if the quantity modification was made before PO date or not.... But I need more experienced ideas than mines 
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Florina Rau