When creating an invoice, or instant booking form, you can set a retainer that splits the amount due. This will require your clients to pay the retainer first. You can schedule the invoice to send with the remaining amount due, or even auto-charge the card for the remaining balance.
You must create a project in order to send an invoice, as the invoices are created and sent within the project.
When you are within your project, click add items or edit invoice, depending on if you created an invoice already.
Click "Options" next to the payment method.
Toggle on the retainer and enter the amount and due date.
You can also set it to auto-charge the remainder on the due date, by switching the charge method from manual to auto.
Press save. You will be prompted to view and send the invoice, or to close it if you wish to send it later.