Double jeopardy. Attorney client privilege. Secure in our papers and possessions. That’s all so 20th century.
Double jeopardy only comes into play when one is cleared in court. One specific branch/department/Law Enforcement agency dropping an investigation doesn’t count.
Not a lawyer, but even if the FBI had "exonerated" Manafort in the past, that is not the same as a trial, so no double jeopardy would apply. If the things they seized from Manafort were authorized by a search warrant, then the judge who issued the warrant must be put on the stand and questioned by the defense, and he would most likely still be upheld.
I too searched for articles on this, but I couldn't find one. This is something I received in an email a while back.