When cancelling a booking, you can issue a credit to transfer any monies back to the customer. Or you can choose to transfer the deposit onto the next booking.
Read this article on cancelling a booking on issuing a credit.
If at the time of cancelling a booking you have chosen to retain the deposit and transfer it to the next booking, choose the credit method as ‘Apply to Next Booking’. This will stop the money leaving your cash list.
Don’t forget to add a note to the customer record to remind yourself next time they book. Go to Customers > Search for your customer > click on change details > Add your note.
When you make the next booking for the customer make sure you select Deposit Transfer within the pick-list.
At this stage it will default to the deposit required for this specific booking, so you will need to check from your customer notes how much the customer paid for their previous booking deposit.
You can also generate a report for any credit raised or deposit transfers made. Go to Reports > Financials > Credits.