Do you understand that this is financial information from when he was a private citizen, not a government official? Without probably cause, this is the very prototype for why the 4th amendment exists.
There is a distinction between a search warrant which needs probable cause and the Congress doing its over sight function which is political. That said there is the over riding issue of privacy. I believe the S.C. found the privacy right to exist in a similar matter involving over reach by Congress some time ago.