You can issue a gift card to the customer to be used on future bookings. The gift card can be issued for over-payment, cancellations or postponement of an event. This can be done by going to the Lead -> Mark Lead cancelled -> Select Issue Gift Card () -> Save -> Email Gift Card to customer.
To email the gift card to the customer, select the Email -> Email Template -> and send to the customer.
If you do not have a Gift Card template set up, please go to your Email Center and search for "Customer - Gift Card" and activate this template. You may customize this template as well.
The balance must be a negative amount to balance out the lead.