Unfortunately, once a booking has been marked as paid, it can no longer be undone. In this case, you have the following options:
- Cancel the booking you marked as paid and create the booking again manually.
- You can also add a comment to the booking, notifying the customer that they still have to pay. You can easily submit a comment by opening the detailed overview of the booking and clicking "Edit".
Everything else you need to know about bookings can be found here.