Suppose, You have a field name free_quantity in sale.order.line and you have a filed with the same name in the account.move.line and you want to pass the field value to the account.move.line.
Solution: You have a function name def _prepare_invoice_line(self) in sale.order.line, inherit it and pass your value in it like,
def _prepare_invoice_line(self):
res = super(SaleOrderLine, self)._prepare_invoice_line()
res.update({
'free_quantity': self.free_quantity,
})
return res
Hope it will help. Thanks