If you are using the Transaction feature, the Transaction Total, and Remaining Budget fields, then you have the ability to trigger a Notification to a relevant person to inform them your Budget Threshold has been reached.
This function uses our Automation feature where you can set up a Rule (for example: when your Budget is within 10% or £100 then notify a particular person via a Task so they can Action if required).
Please refer to the article for Automation Actions that will detail how this particular Rule can be created.