Client Payments: Pre-paying for Services
The earliest you can charge for services is the day the service is rendered. For instance, in the morning, you can manually create your bills and charge for your appointments later that day, but you cannot charge today for appointments scheduled for tomorrow. However, you may pre-pay for services by creating a bill and adding a product.
To begin, go to the client's Billing > Documents tab, click the New... button, and select Bill.

Next, click the + Add Product button. You can add a custom product name and amount, then click the Add button.

After you save the bill, you can go through the client payment process to charge the amount due. Lastly, since you pre-charged for the service, you'll need to go to the client's Billing > Settings tab to add the client credit. When the next client payment is due, the credit will be applied to the payment and the credit amount will be adjusted.