XML Schema Help

Hello All,

I need your help.

In an instance xml file, if I would like to allow different xsi-type and 
just free text like below,

<gemq:priceCode 
xsi:type="gemq:URI">http://www.purl.org/gem/instance/GEMpriceCode/free</gemq:priceCode>
<gemq:priceCode xsi:type="gemq:GEMPriceCode">Free</gemq:priceCode>
<gemq:priceCode>free</gemq:priceCode>

How could I need to change following gemq xml schema?

<xs:schema xmlns:gemq="http://purl.org/gem/qualifiers/" 
xmlns:xs="http://www.w3.org/2001/XMLSchema" 
targetNamespace="http://purl.org/gem/qualifiers/" 
elementFormDefault="unqualified">
<xs:import namespace="http://www.w3.org/XML/1998/namespace" 
schemaLocation="http://www.w3.org/2000/10/xml.xsd"/>

<xs:element name="priceCode"/>

<xs:simpleType name="URI">
<xs:restriction base="xs:anyURI"/>
</xs:simpleType>

<xs:element name="GEMPriceCode">
    <xs:simpleType>
         <xs:restriction base="xs:string">
              <xs:enumeration value="Free Based"/>
              <xs:enumeration value="Free"/>
              <xs:enumeration value="Partially Free"/>
              </xs:restriction>
   </xs:simpleType>
</xs:element>

I treid to do in multiple ways, it keeps generating validation errors.

Could you do me a big favor?

Thanks a lot,

Best regards,
Oknam