Hi Sruthi
As mentioned by Nabendu, variable substitution will not work in your scenario. To be exact, the error "Content is not allowed in prolog" is due to the payload being parsed by the SAX Parser which is expecting XML payload. However, in your case, you would likely have a BIC module to convert the XML to EDI in your receiver channel. Therefore the payload at the point of execution of variable substitution is no longer an XML.
Refer also to a similar error in the following thread.
Re: XML to EDI conversion along with Filename Variable referencing field
Rgds
Eng Swee