This happens at our church, and it probably happens at yours, too. There are times that you want to allow an individual to register online for an event without having to pay the Deposit at the time they register. They will still be required to eventually pay the full fee, but you do not want to require that they make a Deposit in order to register.
Or, you may want to allow them to pay a reduced Deposit, keeping the total fee the same as it is for everyone else.
Help has arrived! We just created a new Ad Hoc Extra Value with special functionality. When you add this special Extra Value to a person's record, they can register online for a specific event and pay either a reduced Deposit or no Deposit at all, depending on what you put in the Extra Value.
This is documented in the Fee Settings article, in the tip entitled "Waive or Lower Deposit Amount".