log message or other to track donations entered as offline cash

Currently the offline cash field is cumulative; i.e., if a team has $20 in offline cash and someone brings in another $5, the team must be edited by an admin and the amount changed from $20 to $25 (if one forgets to add and puts in $5, it changes the total to $5 rather than $25).

Ideally, each offline cash donation could be entered individually with a way to note donor, date, notes, and other info about the donation, and then the donation would be added to existing offline cash. Alternately (and more simply) having the ability to use the log message to note each change in the field (to record where the donations came from) would help with the organization's bookkeeping and ensure that donations are not duplicated or missed, and allow some path to track and reconcile the offline cash.

Note: originally posted as http://drupal.org/node/1160444 (closed that issue as a duplicate of this one in order to separate two different feature requests), and changed to request against 7.x version.