Mueller wants to "expand" his investigation into Trump's finances. He knows the Russian collusion bullshit is going nowhere, so he has to find something else to justify the continuation of the his bogus investigation.
I'm pretty certain you have no clue as to the powers and duties of Mr. Mueller; you've proved to be a very biddable (easily led) individual since you have no real evidence that Mr Mueller is conducting a "bogus investigation".
It is not a leap to understand that any financial matters Trump may or may not have engaged in with Russia, or with Russian Oligarchs anywhere in the world, would be hidden by brokers , or transactions in banks not chartered in Russia or the United States. His taxes, once promised and never produced strongly suggests they have evidence germane to Mr. Mueller's investigation.