Dedcuct from Retainer before sending Invoice

Hello One and all

I just started re-using zoho invoice for my OneMan IT Shop.
I make use of retainer payments in the form of pre-paid services.

What i have noticed, is when i generate and invoice, i have to
1. Save it first
2. Deduct the Invoice amount from the Retainer
3. Then finally send it off to the client

What would be much more idea.
1. Invoice is created.
2. Zoho Checks to see if there is funding in the Retainer account
3. Just below the invoice, Zoho, shows u how much is in retaner
4. Zoho, then shows u a [ ] check box, asking you if you would like to deduct the invoice amount from the retainer
5. Once you click the check box in agreemeent, you then send the invoice off t the client
which will then indicate the amount of the invoice, the amount deducted via retainer, and then the balance.

This will all be done in on step, and not 3 steps as it is currently.
This saves lots of time