A midsize law firm found a way to deal with an aggressive outside auditor hired by a client to review its legal bills — tell the client to find new lawyers.

After Law Audit Services Inc. used a failure by Manhattan’s Armienti & Brooks to meet stricter billing rules as a reason to cut more than $100,000 from invoices totaling $400,000, the law firm decided it was no longer worth handling cases for the client, Brooklyn-based Empire Insurance Group, and made arrangements to send back 400 open matters.