A customer can book in the booking system a certain amount of time (like 10 hrs).
If the 10hrs has been booked. The customer then can buy more booking time. Or if the customer books for only 5hrs at the moment. The 5hrs left (total 10hrs - 5hrs = 5hrs). can be booked later on another time.
The time can be used for any service provided in the booking system.
I'll hope this feature gets in :)