1. Create a new QWEB view that inherits and overrides the standard Invoice report.  Do this via Settings --> Technical --> User Interface --> Views

2. Add the following to the ARCHITECTURE FIELD:

<p t-if="o.comment"  position="before">
  <t t-set="paid_total" t-value="- o.amount_total + o.residual"/>
  <div class="row">                
    <div class="col-xs-4 pull-right">                    
      <table class="table table-condensed">                        
        <tr class="border-black">                            
          <td><strong>Less Payments</strong></td>                            
          <td class="text-right">                                 
            <span t-esc="paid_total" 
                  t-options="{&quot;widget&quot;: &quot;monetary&quot;, &quot;display_currency&quot;: o.currency_id}"/>
          </td>                        
        </tr>
        <tr class="border-black">                            
          <td><strong>Open Balance</strong>
          </td>                            
          <td class="text-right">                                 
            <span t-field="o.residual" 
                  t-options="{&quot;widget&quot;: &quot;monetary&quot;, &quot;display_currency&quot;: o.currency_id}"/>
          </td>                        
        </tr>  
       <!-- OPTIONALLY INSERT WORDS HERE --> 
      </table>                
   </div>            
  </div> 
</p>

3. Make sure RELOAD FROM ATTACHMENT is not checked in the Report Definition.  Find it at Settings --> Technical --> Actions --> Reports.

4. The Invoice Report will then look like this:

EDIT: To add the words PAID, add this at the end of the XML in the ARCHITECTURE field (where is has <!-- OPTIONALLY INSERT WORDS HERE -->)

<tr>            
  <td>                
    <span t-if="o.residual == 0">PAID IN FULL</span>            
  </td>        
</tr>