A lot of businesses collect an advance payment (or retainer) for products sold or services rendered by them. This amount collected will not be an income but a liability to the company. The revenue is earned only when the product is delivered or the service is completed, if not delivered or completed the advance payment made will be returned to the customer.
This retainer can be invoiced to your customer using the Retainer Invoice module in Zoho Billing. Learn more about how Retainer Invoices can be used in Zoho Billing.