If someone has already registered then you are able to change that individual's amount owing. It can be used to manually adjust fees owed/no longer owed due to price adjustments or any other reason.
Only the base fee can be changed and it cannot be set to anything less than 0.
This is different from creating a new invoice. Changing a participant's fees will change their amount due, which adjusts how much is asked for when requesting payment and changes their amount due when viewing a list of participants.
To edit a participant's fees:
Use the season search in the top right to find them by name or go to the registration form's participants
Click edit or the participants name to edit them
Click the edit button beside the fees that you want to change
Press save at the bottom of the page when finished with changes