This is an illogical request. If the FBI was the least bit thorough in their raids of Cohen's offices, they should already have that data.
Even though the press release says they are investigating Cohen's private business deals, all they looked for was everything and anything related to President Trump. They probably forgot to look for things related to the "fake reason" that they were doing all of this in the first place.
The whole thing is another Mueller 'fishing' expedition. And it was, if not illegal, it was certainly a very, very, dubious thing to do. To extort an attorney to turn on his confidant client, wow. That is out of bounds for almost everybody, in America that is, except Mueller and his fanatical pro-Hillary, pro-Democrat, team.