This guide demonstrates how to add a temporary reporting period which will enable you to add a payment on account as an opening balance.
Navigate to the Settings > Accounting > Periods menu.
Add a start and end dates to include the date of the payment on account. Click on the Add button.
Navigate to Sales > Money in.
Fill in the details of the payment form and hit the Confirm button.
Navigate back to Settings > Accounting > Periods menu.
Delete the reporting period that you created in step 1.
Navigate to Dashboard > Getting started > Cash.
Adjust the opening bank balance to reduce the balance if appropriate and hit the Update button.