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 allows your customer to have flexibility, personalization and it's at their convenience on when they want to reschedule. This can be done by going to the Lead -> Mark Lead cancelled -> locate where the balance is and Select Issue Gift Card -> Save. Once you save the lead, the gift card will automatically email to the customer (if you have this email activated). 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. See this article for more information pertaining to gift cards.