sudhir,
My second point was to do two steps of configuration:
1. in t-code VOV8, in your sales document type, give the delivery block in the field "Delivery block".
2. assign the delivery block to the Sales document type. The path is IMG>Logistics Execution>Shipping>Deliveries>Define Reasons for Blocking in Shipping>Delivery Blocks
By this when sales order is created, automatically delivery block is put in the order.
After payment is done, the user can manually remove the delivery block from the order (in VA02).
Note: As you said that for every sales order a proforma is created. Then by default the delivery block can be put in every sales order. (A separate action can be to create the Proforma invoice)
If for every sales order, a Proforma invoice is to be created immeidately; then the system can also handle this with output determination.
Is your requirement to create Proforma invoice manually?