The Kendo Scheduler does not have built-in functionality that would allow you to perform server validation and checks before saving an event. Having that said, in order to achieve the desired, you will need to implement your own logic that prevents the default execution of the Scheduler save event, sends an AJAX request to the remote endpoint, performs the required check, and if that check passes, saves the event. Here is a Knowledgebase article demonstrating such an implementation:
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