If you're trying to record an opening balance for employee reimbursement that has been paid already, you can create a year-to-date adjustment. I'll show you how.
Go to Employees on the left pane and choose the Employees tab.
Click on the employee's name you wish to enter YTD figures for.
Go to Pay history.
You will notice all the fields are inactive. Just click on Make Adjustments to make the field active and available to change.
Add the YTD values in the Adjustments column, for both sections; This Employment and National Insurance.
Once you're done, click on Save.
You can also check out this article for the detailed steps in entering year-to-date figures in QuickBooks Online (QBO).
Otherwise, you can create a journal entry if it's still payable to your employee. I'd recommend reaching out to an account for better guidance in recording the form.
We're a post away if you need anything moving forward.
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