Hard Boiled Mahoney (1947)

They're tip-top snoopers in the tip-toe racket!

Sach just lost his job as an assistant to a private detective, but he wasn't paid. Slip goes with him down to the detective's office to demand payment, but finds the office empty. A woman enters the office and mistakes Slip for the detective and convinces him to take on a case to find her sister after offering a $50 retainer.