How to change value of DynamicXML in a legacy application

I am to manipulate and add a feature to a legacy application that heavily on XML and dynamic values.

I simply what to update the value if a condition is met.

      public class DynamicXml : DynamicObject
    {   
         XElement _root;
        private DynamicXml(XElement root)
        {
            _root = root;
        }

          public static DynamicXml Parse(string xmlString)
        {
            return new DynamicXml(RemoveNamespaces(XDocument.Parse(xmlString).Root));
        }

         dynamic dynamicXML = DynamicXml.Parse(data);

           if (dynamicXML.POLICY.PARTNER == "Ecom")
              {
                 if (dynamicXML.POLICY.MARKETER == "Dressing")
                     {
                        dynamicXML.POLICY.PRICE = "1800.00";
                     }
              }
           }

<POLICY>
    <MARKETER>Dressing</MARKETER>
    <PRICE>1000.0000</PRICE>
    <PARTNER>Ecom</PARTNER>
</POLICY>

I keep getting {"'Middleware.Models.DynamicXml' does not contain a definition for 'PRICE'"}