Go to Payables > Transaction Group Status List to find the status of the transaction.
If the transaction is awaiting authorisation (Status = Confirmed or Partially Authorised)
An Authoriser can reject the transaction:
If the Authorised transaction is awaiting processing (Status = Authorised Scheduled)
The user who has authorised the transaction will need to remove their authorisation:
Once authorisation has been removed, an authoriser can reject the payment:
If the transaction has been Processed (Status = Approved, Authorised Released, Processed, Sent For Approval or Sent For Processing)