For pro-bono work, you can manually assign those clients' credits to the number of appointment types you have agreed upon. This way, they will be able to book only the amount of appointments you have given them.
You can do this on a client by client basis by selecting Billing, and then the Client Packages menu, then selecting "Manage Client Credit" in the top right-hand corner. This will require that the client already exists within Healthie.
This Manage Client Credit feature is particularly useful in combination with the "Prevent No Credit Booking" setting enabled. You can find Prevent No Credit Booking in Settings, and then Appointments, and then by scrolling all the way to the bottom of the page!