Retainer Payments

Hi Forum members,
In my line of business, i prefer to do pre-paid projects, cause you are 100% sure of your revenues..

Do Zoho have any plans to add a function for Retainer Payments?
Is there any work around that i can use at the moment?

How does Retainer payments work.

A client is invoiced for $100.00 for 20Hours of work, and he pays that invoice.
For every hour of work that is down, it's deducted from that $100.00 dollars, and the client sees this back in the reports
When the $100.00 is deplited, a new Retainer invoice i sent to the client.

Any assistance that can be granted, i'll be happy to hear,.

Thanks
Reggie