If they are looking into Trumps business dealings (and tax returns) before he ran for election, then that cannot have anything to do with Russian interference during the election. It really is that simple.
I'll ask the same question I posted before. Where did you ever get the idea that Mueller and his team are looking into Trump's business dealings before he ran for election? Let me guess ... from some media report based on "unnamed sources close to the administration" (or some other such nonsense) that you read on the internet in the last couple of days.
These people in the media are jerking you around, and you don't even realize it.