Hi Carlos,
What action you are performing when the user click on reject button? are you just sending an email to PO creator for doing modification ? if so and you want the workflow to be triggered when he does any changes, then you can insert event 'Changed' of business object FIPP in workflow start condition (SWDD->basic data setting) alongwith CREATED event which you have right now. doing so workflow will get triggered upon doing any change in the document.
But doing this workflow will trigger for any change in the document and then there could be multiple workflows in 'IN PROCESS' state for same document. To overcome this you need to create a CHECK FUNCTION MODULE also. and insert it in event linkage (tcode SWE2) of CHANGED event of FIPP.
in this check FM, check if there is already a workflow for particular FI doc (use FM SAP_WAPI_WORKITEM_TO_OBJECT) in process. if so raise an exception and new workflow will not trigger !!!
you can refer below link for info. on check function module .
http://****************/Tutorials/Workflow/checkfunctionmodule/demo.htm
http://scn.sap.com/thread/3383394
Regards,
Ibrahim Ansari