How to Apply retainers automatically to scheduled regular invoices when sent

How to Apply retainers automatically to scheduled regular invoices when sent

I take retainers for my business to confirm a service date. 
Once the client has accepted the estimate, a retainer invoice is automatically created, and the estimate is automatically changed to a draft invoice.
I can then schedule the draft invoice to be sent, but I cannot automatically apply the retainer amount to the invoice.
If I apply the retainer amount before I send the invoice, the invoice is changed from draft to sent, and the option to schedule the email goes away. In addition, applying the retainer early recognizes that retainer income in a different period than the service date and final invoice which is not desirable since the retainer is often paid months in advance of the service date.

Is there a way to automatically charge the retainer to the invoice when it is scheduled to be sent?