In Amendment for Sale and Purchase - #6 by ced you mention that you don’t want to manage certain things in the amendment.
But to ‘create’ the missing ‘line’ first, in order to change the quantity, why is it impossible to not simply invoke the form for purchase.lines.new() forcing quantity=0 in order to do such? It already displays such a form but with everything greyed out.
By the way, the amendment also seems to omit updating the delivery date.