Billing based on time blocks.

Have a question. We are a MSP and have customers buy blocks of time and when we do work we create tickets and the ticket shows how much time they have and we put in how long we worked on that ticket. It takes away from that side. Example customer buys 20 hours we work on it for 4 hours then 6 it shows 10 hours used customer has 20 so 10 left. Any thing like that in your system? I saw the timer it seems close but I have to run reports or see totals for customers and where the time went.

If anyone knows of something like this for simple billing would be great.