Hi Raveendhiran,
Yes, it is possible to fetch sales order net value. But why you need the sales order value on invoice layout ? is it just for statistical value ( to display) ?
However, you can check in the Print Program. Definitely there would be VBAK table declared in the program. directly fetch VBAK-NETWR value
If VBAK is not declared in the print program, then follow as suggested by C.B.Reddy
thanks
santosh