Under Admin / Defaults / Manage System Defaults is an option "Prompt for a reason when deleting"
Once this is ticked, when any pending transaction is deleted a box will appear asking the user for the reason why this is happening.
It is essential that users enter the correct information in this box, if a payment has been assigned to the incorrect account, a pending invoice or purchase order deleted. Having the genuine reason this has been done will assist if it needs to be followed up on.
These explanations are recorded in Admin / Other / View Audit Trail. Selecting Details will show the reason entered.
See How do I Find a Deleted Transaction or Item for more on the Audit Trail.