Safe way to cancel shrinking hard drive partition
Unless there is a  Cancel button that, when pressed, gives a warning that it will revert the current action, wait it out.  

If there is no Cancel button, then you have no choice but to wait.
If you force it to stop halfway through, your data will be in an uncertain state.

BTW running a backup to the same storage medium is rather pointless... if the drive goes down it takes your backup with it.