Retainer billing category
TimeCache offers a special billing category type that is useful for people who have retainer arrangements with clients. The Retainer type lets you add entries to track the amount of a retainer for a project/matter. To create a Retainer category, simply click the Retainer button in the Category Info window.

Then you can enter retainer billings or payments for individual projects on the Daily Log, and apply time and/or expenses to the retainer.