CiviRules is available and you can use it to do what you want. There is documentation as well :-)
If you want to use CiviRules to send an email you will also need the send email API (and please pull the civirules branch): https://github.com/CiviCooP/org.civicoop.emailapi
Once you have that you create a Rule, select the trigger for the rule as an event and come up with conditions. In your case the event will probably be 'new contribution' and the condition might be the source or the financial type, depending on how you are going to distinguish a back end contribution.
The action will then be 'send email', or could possibly also be 'create an activity'. It would make sense to me not to send lots of internal email but instead to create a specific activity type with a dashboard so the user can see his daily workload. But perhaps your end users have other preferences.