In general, under the cash basis of accounting, transactions are only recorded when there is a related change in cash. This means there are no accounts receivable or payable reflected on the balance sheet.
Based on your facts, you have not had that change in cash.
However, if this 401k is an allowable expense for the applicable 12/31 year, then I would reflect it on the balance sheet to minimize any confusion.
Reflecting this on the balance would minimize the complexity of having to record a book to tax adjustment for this two years back to back.
Not a big deal.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
