How to charge a customer of the remaining balance at a later stage?

It is possible to charge a customer with the remaining balance directly from HBook only if you have Stripe gateway activated.

In HBook->Payment->Stripe settings, there is an option "Store credit card". If you set it to "Yes", the customer credit card details are saved on Stripe. 

In Reservations, you will then be able to charge the credit card at a later stage by clicking "Charge" in column "Price/Payment".