Hi Adrian

First, disable trailing slash in umbracoSettings.config:

  <!-- this will add a trailing slash (/) to urls when in directory url mode -->
    <addTrailingSlash>false</addTrailingSlash>

Second, add redirect:

<rewrite>
  <rules>
    <rule name="Remove slash" stopProcessing="true">
      <match url="(.*)/$" />
      <conditions>
        <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
        <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
      </conditions>
      <action type="Redirect" redirectType="Permanent" url="{R:1}" />
    </rule>

  </rules>
</rewrite>

Thanks,

Oleksandr