Hi Ajay and all,
It is resolved with the suggestion from Ajay. It worked out with implicit enhancement. Here not only sales order, we also derived sales order item.
As Ajay suggested, it will be a problem if one WBS is assigned to different sales order. In this case system is going to pick up the 1st sales order by default. In our particular case, my client is going to have only one sales order for one WBS element.
We have gone for positive and negative testing, it is working fine.
But the exit i.e. COPA0001 was not helpful.
Just ABAPer uses the table VBAP and does the coding required.
Thank you very much Ajay....
Sanhhitha