copy pricing values from quotation to billing

Hi,

our requirement is to copy the values of pricing elememts from Quotation to billing document ( delivery related)

Business scenario

1. Create quotation

2. Create S.O with rererence to Qtn.( price may be changed in S.O)

3.Create Proforma Invoice

4.Create delivery

5. Commercial Invoice - while creating Invoice values should be copied from Quotation as the values are changed in the S.O

In copy control we can maintain pricing source as sales order, delivery,P.O. we do not have any other options.

can you please suggest options to meet the requirement.

Thanks

Regards