You can use XSLT mediator or JavaScript
A sample proxy with javascript doing what you want :
<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse"
name="TestSOF"
transports="http"
startOnLoad="true"
trace="disable">
<description/>
<target>
<inSequence>
<property name="FORCE_SC_ACCEPTED" value="true" scope="axis2"/>
<property xmlns:open="http://www.openuri.org/"
xmlns:ns="http://org.apache.synapse/xsd"
xmlns:poin="http://eai.mtn.iran/PointOfSales"
name="Operation_Name"
expression="//poin:Operation_Name/text()"
scope="default"
type="STRING"/>
<property xmlns:ns="http://org.apache.synapse/xsd"
xmlns:poin="http://eai.mtn.iran/PointOfSales"
name="PointOfSales"
expression="//poin:PointOfSales"
scope="default"
type="STRING"/>
<script language="js"><![CDATA[
mc.setPayloadXML(<open:{mc.getProperty("Operation_Name")} xmlns:open="http://www.openuri.org/">
{new XML(mc.getProperty("PointOfSales"))}
</open:{mc.getProperty("Operation_Name")}>);
]]></script>
<log level="full"/>
</inSequence>
</target>
</proxy>
A sample using XSLT :
<?xml version="1.0" encoding="UTF-8"?>
<localEntry xmlns="http://ws.apache.org/ns/synapse" key="SOFXSL">
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:fo="http://www.w3.org/1999/XSL/Format"
xmlns:fn="http://www.w3.org/2005/xpath-functions"
xmlns:open="http://openuri.org/"
xmlns:env="http://eai.mtnn.iran/Envelope"
xmlns:poin="http://eai.mtn.iran/PointOfSales"
version="2.0">
<xsl:output method="xml" encoding="utf-8" indent="yes" omit-xml-declaration="no"/>
<xsl:param name="operationname"/>
<xsl:template match="/">
<xsl:element name="open:{$operationname}">
<xsl:apply-templates select="//poin:PointOfSales"/>
</xsl:element>
</xsl:template>
<xsl:template match="@*|*|comment()">
<xsl:copy>
<xsl:apply-templates select="@*|*|text()|comment()|processing-instruction()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
<description/>
</localEntry>
<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse"
name="TestSOF"
transports="http"
startOnLoad="true"
trace="disable">
<description/>
<target>
<inSequence>
<property name="FORCE_SC_ACCEPTED" value="true" scope="axis2"/>
<property xmlns:open="http://www.openuri.org/"
xmlns:ns="http://org.apache.synapse/xsd"
xmlns:poin="http://eai.mtn.iran/PointOfSales"
name="Operation_Name"
expression="//poin:Operation_Name/text()"
scope="default"
type="STRING"/>
<xslt key="SOFXSL">
<property name="operationname" expression="get-property('Operation_Name')"/>
</xslt>
<log level="full"/>
</inSequence>
</target>
</proxy>