You can cancel a pending transaction by coming to support the day it was created (i.e. when plum announces the saving) and ask support to cancel it. You can do this by typing 'chat to human' in Plum messenger (or if using the app tap 'help')
We are able cancel before 3pm that day afterwards you will be able to withdraw when you saving reaches your Plum account.
If you made a manual saving in the afternoon, we can cancel it by 3pm the next day