You can create a field in account.move and update its value from sale.order then after you can easily access its value in invoice report. i.e: 

class AccountMove(models.Model):
        _inherit = "account.move"
        delivery_date = fields.Datetime("Delivery Date")

class SaleOrder(models.Model):
        _inherit = "sale.order"
        def _prepare_invoice(self):
                invoice_vals = super(SaleOrder, self)._prepare_invoice()
                invoice_vals['delivery_date'] = self.commitment_date
                return invoice_vals

In this example: i am updating the custom field delivery_date of account.move by using _prepare_invoice() method.