M client has paid me some funds in advance. How do I handle a prepayment for invoices I will be submitting to this client?

Hello rick84, 

Welcome to the Community! QuickBooks Online makes it easy to record payments received from your customers. Determining how to record a customer payment that's made prior to issuing an invoice can be confusing, but don't worry, I can help shed some light on how to go about this. 

The payment can be recorded through the Receive Payments screen like usual, but without attaching it to an invoice. This creates a credit on the customer's profile which can be applied to a future invoice. Here are the steps to record an advance payment:
 

• Click the + New button towards the top left.
• Select Receive Payment.
• From the Customer dropdown, select the name of the customer.
• In the Payment method dropdown, choose the payment method.
• Use the Deposit to dropdown to select the account you want to put the payment into. 
• Enter the payment amount in the Amount Received field. 
• In this situation, don't select any transactions within the outstanding transactions list. 
• Then select Save and Close. A warning will appear saying that a credit will be created since an invoice was not selected. Click OK to continue. 

After following these steps your payment will then be recorded. You can read more about recording payments in this article: Record invoice payments in QuickBooks Online

Please let me know if there's anything else I can assist you with!