You need to file a lawsuit against the previous manager: subpoenas (or other "discovery" mechanisms, or ways to get information, such as written interrogatories [questions] or document requests) can only be issued by a private citizen in the context of a lawsuit or legal action, and then only if the documents or information requested is/are relevant to the suit in some way. So if you were to sue the previous manager, you could enforceably request the documents from him (such as with a document production request) and/or supoena the bank. You'd need a basis to sue him, of course, such as breach of contract (not doing or providing everything he'd agreed to), or fraud (lying about something important, like the business financials), or theft (taking business records).