When a retail sale is made, it gets added to the customer's account. You can remove this item from the account if it was sold incorrectly, or you need to make an adjustment.
If you are not on the customer's account, navigate there by clicking on 'Accounts' and searching for the account or customer in question:
When you click on the line of the account you want to open, you will see any appointments, retail sales, and payments listed on this account:
Near the top right of the screen, click on the button that it titled 'Add / Delete Retail':
Click on the 'trash' icon for the retail item you want to remove:
When this is done, the retail item is removed from the account and the inventory level of your retail stock is increased by the quantity that was removed.