No problem!
There’s a previous post on this here: Unpaid invoices and trial balance.
Basically, whatever the invoice is doing, you would reverse it on the trial balance import so it has no effect on your opening balances.
So if the invoice is for £500.00 to 4000 General Sales, and £100.00 VAT, you would reduce your trial balance amounts by this. Once you’ve made any changes, check your balance sheet for the opening balance date against your trial balance, and it should all match up.
Any problems, please let us know - we’re here to help!