Well, What you have done so far? I am trying to describe general steps here: (I assume you are using orchestration)
For your easiness you can Create a distinguished field for e.g. field1 in your case to check the condition within the loop. The loop will have one record at a time if you have multiple records. based on the condition result you can create/change XML message.
The alternative is to try to read the value of field directly using xpath. You can also change the XML after checking condition. To know using xpath please refer these posts:
http://geekswithblogs.net/sthomas/archive/2004/10/25/13269.aspx