Hello, sr1962dl.

I'll lend a hand and ensure you'll be able to record the client's overpayment. I'll also add some details regarding the VAT element. 

You'll simply want to record the overpayment through the Receive payment function. There's no need to account for the VAT element since it's already calculated through the client's invoices. 

To record the overpayment: 

1. Click the + New button, then select Receive payment. 
2. Choose the client's profile under Customer.
3. Set the Payment date to when they paid those invoices. 
4. Under Deposit to, select the bank account or the Undeposited Funds account where you want to deposit the money. 
5. For the Amount received field, enter the total amount that the client paid (the exact amount that leads to the overpayment). 
6. Go to the Outstanding Transactions section, then check the invoices that the client paid. 
7. Once done, click Save and close. 

Below the invoices table, you'll see the Amount to Credit section. QuickBooks will automatically calculate this and put a credit on the client's profile.

You can then use the credit amount for future invoices (through the Receive payment function again). 

You can use this article if you need more help recording your invoice payments: Record invoice payments in QuickBooks Online. 

Need to reconcile your transactions within QuickBooks Online? We have an article to help you with the process: Reconcile an account in QuickBooks Online.

You're more than welcome to ask any other questions about recording other transactions (or doing a specific task in QuickBooks). I'll get back to this thread and lay down the steps and details.