Let's work together to sort out the credits on your account, RMA711.
Since you've issued a check to your customer while there's already a credit available on their account, we need to make some adjustments to correct the balance. I'll outline the detailed steps below:
1. First, locate the customer by going to the Customer menu, then click Customer Center.
2. Double-click the check to open, and ensure Pay To Order Of has the correct customer.
3. In the Expense tab, enter the amount, select Account Receivable in the Account column to zero out the credit, and then click Save and Close.
4. Return to the Customer's Information window, double-click the Invoice, and select the Receive Payment at the top.
5. From the Customer Payment window, checkmark the issued check amount to link them and correct the credit balance.

I'm also sharing this article to help you review and ensure your accounts are balanced and accurate: Reconcile an account in QuickBooks Desktop.
I'll be around to help whenever you have additional credit concerns. Take care.