There are plenty of time trackers, but it’s hard finding one that supports billing out of a ‘bank of hours’ like you describe. Once I ended up writing a script that would work with my time tracker API to do this because I couldn’t find a better tool. :\
Eventually, I started using Freshbooks’s estimate features, which allows you to set a cap on hours for a project. Then, whenever a new block is purchased, I add those hours to the project. Makes it easy to see how much time is remaining for each client.