[FIX] account: display negative value in portal

The residual amount of a customer invoice or a credit note is displayed
in the same way in the portal view. This can be confusing for the end
user since there is no way to distinguish both.

We display the residual amount as negative in case of a credit note.

opw-1841432

addons/account/views/account_portal_templates.xml

@@ -57,7 +57,7 @@
                                 <span class="label label-default"><i class="fa fa-fw fa-remove"></i><span class="hidden-xs"> Cancelled</span></span>
                             </t>
                         </td>
-                        <td><span t-field="invoice.residual" t-options='{"widget": "monetary", "display_currency": invoice.currency_id}'/></td>
+                        <td><span t-esc="-invoice.residual if invoice.type == 'out_refund' else invoice.residual" t-options='{"widget": "monetary", "display_currency": invoice.currency_id}'/></td>
                     </tr>
                 </t>
             </table></div>