How do I enter the client's retainer and then generate an invoice/bill that shows the work that was done and how much of the client's retainer is remaining?

SUGGESTED

So my attorney charges her clients a retainer fee based on the type of case she is handling (so each client can have a completely difference retainer amount), and on top of that, she is billing AGAINST the retainer. We were generating bills in Word format that showed what my attorney was billing for and how much of the client's retainer remained after each month of work on the matter. My question is: Is this possible as well in TimeSlips? I don't want to generate a bill and have our clients thinking that they owe this money, but I do want to generate a bill that shows the client their current retainer balance. 

And for reference, we have the new TimeSlips Pro that we just had installed on our system this month. 

Thanks in advance for any help!