Deleting an unpaid bill from 2020 will increase your accrual net income with no effect on cash basis income and, unless you were a very busy beaver and have already filed your 2020 tax return go ahead and delete or void.
If this had been from a closed AND filed period, say, 2019 or 2018, the procedure would be to leave the bill in place but create a current year credit memo reversing it. It is important to not affect filed taxes, although a big enough change in your favor would command an amended return. But then it comes down to whether you report as cash or accrual. Unpaid bills or invoices are disregarded under Cash accounting unyilnpaid
