venu,
When creating Proforma invoices, the value of this field is blank.
Proforma invoices are not "real" invoices.
During creation of proforma invoices, the system does not check the preceding document.
If you are changing the value of this field to blank, in VTLA, then it is incorrect.
As the would not check the preceding sales order, during creation of delivery notes.
Test, see what is the value of field "Delivered quantity" in Tab Schedule lines, in the sales order; when field Pos/ neg quantity = blank, in VTLA.
I think this field "Delivered quantity" would be blank.
If needed, I shall test this too.
But first please try testing this yourself.