I'm trying to figure out how to customize an Estimate to include a Deposit/Retainer Invoice line within the Estimate. 

I tried creating my own custom field called "Deposit" where I manually inputted the deposit amount, which was nice. However, my employer would like for the estimate to reflect the sub total, deposit amount, and amount left if deposit was paid. Kind of like this:

Total: R4.900.00

Deposit: R2000.00

Total Due Upon Completion: R2900.00

I understand that there is a separate area for sending out Retainer Invoices (as well as a button to autosend it once the Estimate is sent). I don't want them to be able to pay through the Estimate, but I want to give them a detailed look at what the Retainer Invoice will request once sent, as well as what the remainder payment will be once they pay it.

Does anyone know if there a way for me to do this within Zoho Books or at least something similar?