Получить разницу двух переменных в XSL

<xsl:with-param name="x" select="format-number(property[@id=204]/value, '### ##0', 'my')"/>

<xsl:with-param name="n" select="format-number(price_discount, '### ##0', 'my')"/>

<xsl:value-of select="$x - $n"/>
.
Пробую все вывести вот так
<xsl:choose>
                           <xsl:when test="property[@id=204]">
                              <xsl:call-template name="sum">
                                 <xsl:with-param name="x" select="format-number(property[@id=204]/value, '### ##0', 'my')"/>
                                 <xsl:with-param name="n" select="format-number(price_discount, '### ##0', 'my')"/>
                                 <xsl:value-of select="$x - $n"/>
                              </xsl:call-template>
                           </xsl:when>
                           <xsl:otherwise></xsl:otherwise>
                        </xsl:choose>

<xsl:variable name="x" select="format-number(property[@id=204]/value, '### ##0', 'my')"/>

<p>   
<xsl:if test="price_tax != price_discount">
    <xsl:variable name="price_tax" select="price_tax"/>
    <xsl:variable name="price1" select="price_discount"/>  
            <xsl:value-of select="$price_tax - $price1"/>         
   </xsl:if>         
               </p>