Resolution:
In order to publish data containing €' (the Euro), Please validate the following:

1. Set the adapter encoding to UTF-8. (Under Siebel Adapter instance, goto ‘Configuration’ tab, check ‘Show All Tabs’ and under ‘Adapter Services’, set the encoding to UTF-8).

2.  Set the  project encoding to UTF-8. (Open project in designer, go to File --> SaveAs and set TIBCO Messaging Encoding to UTF-8.)

Note: If the project is deployed using TIBCO Administrator, make sure that the following two parameters in the admin tra file (present in tibcoadmin_&ltdomain name>.tra in Admin bin folder) are set to UTF-8:    
1. repo.encoding
2. tibcoadmin.client.encoding    
After you have changed these property values to UTF-8, you will have to restart Admin and redeploy the project.