Hi Debashis,

First for transfer of any fields from one model to another create the field in both model here in your case you have create a field in sale.order so again create the same type of field in account.invoice table.

after that pass that value from sale.order to account.invoice, if you seen in the sale.py file you will see a def function while for creating invoice and values :

def _prepare_invoice(self, cr, uid, order, lines, context=None):

enter code here over here you may pass ur value which will go n store in fields define in account.invoice table ex . if you created a field in sale.order x_workorder create also field in account.invoice after that come in this def function and inside :

invoice_vals = {
                'yourfieldinaccount.invoice': order.x_workorder ,
               }

then you find the value in invoice as well and yep use proper xml too.

Hope this will help .