Currently, the option to convert a sales receipt to an invoice is unavailable. You can delete your sales receipt so that you can make a receive payment on your invoice for this customer. Let me guide you how:
On the left pane, select Sales then choose Customers from the drop-down list.
Find and select the customer's name.
Under the Transaction List tab, locate the sales receipt.
Click the drop-down arrow on the Action column and select the Delete option.