I have done some further investigation. Since we have no workflow there is nobody who has to approve a leave request. Every leave request which gets created is directly set to Status POSTED. There was a post about about withdrawing leave requests which makes me think if it is in general possible to withdraw an already POSTED leave request without triggering an extra workflow. So my question is, is it possible to withdraw or delete an already POSTED Elemente with My Leave Request V2 App and it' oData service. In Dynpro it's working, but after debugging it seems the oData service doesn't allow actions like this.
Thanks stefan for answer -> we included note, but its not working after all
Here the post -> Fiori Leave Request - Withdraw