There used to be a way to add notes to payments and waived fines, but that's not possible anymore. We need a way to record why we're waiving fines for auditors and accounting reasons.
This may be related to bug 6413, but I don't know how so I'm not putting it in depends or blocks.
I just tried and the payment note is also recorded when you waive a single fine. Is this what you were thinking of?
Updating the bug report, it works most of the time, but not when using "Write off all".
*** Bug 10260 has been marked as a duplicate of this bug. ***