Hi Sharon 8,

You can create a Delayed Credit to record the credit, then apply this to your invoice. This shows the credit and the amount of the sale, with the total owing showing as 0:

1. Select +New, then Delayed Credit.
2. Select the customer, date and so on. Enter the details of the credit and select Save.
3. Select +New, then Invoice.
4. Select the customer, date and so on. From the column on the right, Add the credit. 
5. On the second line in the invoice, enter in the sales information. You can select Print or Preview down the bottom prior to clicking Save and Send.

You may also consider using Sales Receipts to record the sales in future - see this resource on using sales receipts vs invoices if required.

-Kass