Manafort's legal team has argued that Rosenstein inappropriately gave the special counsel's office an unlimited "blank check" when he authorized Mueller last May to investigate any matter that "arose or may arise" from an examination of the Trump team's ties to Russian Federation.
The revelation of the August 2 memo comes amid a broader court filing from Mueller's prosecutors that offers a full-throated defense of their investigative powers and indictments thus far.
U.S. District Judge Amy Berman Jackson of the District of Columbia made clear her skepticism as she questioned Manafort attorney Kevin M. Downing. He also asked the judge to bar Mueller's potential future actions in Manafort's criminal cases.
The government will tell the judge that Manafort has no right to challenge how the Justice Department's rules governing special prosecutors are enforced.
Downing faced an uphill battle Wednesday morning trying to persuade Jackson that Manafort can challenge the appointment order under the Administrative Procedure Act, which governs how federal administrative agencies propose and institute regulations.
Special counsel Robert S. Mueller III was authorized by a top Justice Department official to investigate whether Paul Manafort, the onetime chairman of Donald Trump's presidential campaign, illegally coordinated with Russian Federation to interfere in the 2016 election, new court filings show.
Although Manafort's lawsuit originally asked the judge to invalidate the entire appointment order, Downing clarified that they were now only asking the judge to set aside the section referring to "any matters" that came out of the investigation. At Wednesday's hearing, Downing also said that he was no longer pursuing the second count - focused on Mueller himself - and that the lawsuit was just challenging the appointment order.
Downing said he didn't contest that Mueller could investigate possible links between Russian Federation and the Trump campaign in the 2016 campaign, but he said Mueller shouldn't be able to also examine "any matters that arose or may arise directly" from his probe.
Downing later said he did not know if he had such a case.
The criminal indictments against Manafort DC and Virginia state that Manafort's political consulting firm, Davis Manafort Partners Inc., had staff in Russia, Ukraine, and the United States. Manafort has not been charged with any crimes connected to the presidential race.
Previously, Manafort's attorneys complained that Mueller exceeded his authority by opening cases against Manafort, because they involved business conducted years before he joined the Trump campaign in 2016. A version of the memo filed in court showed that Rosenstein signed off on an investigation of whether Manafort "committed a crime or crimes by colluding with Russian government officials" and of Manafort's work as an global political consultant in Ukraine before joining Trump's campaign.
The Justice Department didn't file that memo in Manafort's civil case, but Jackson asked Downing and the Justice Department about its significance on Wednesday. Justice Department lawyer Daniel Schwei brushed off the memo, saying it didn't have any bearing on the purely legal arguments about why Manafort couldn't bring a civil lawsuit. He said Manafort should address his indictments in his criminal cases.
Schwei argued that regardless of Manafort's efforts to narrow the focus of his civil lawsuit, it would "plainly interfere" with the ongoing prosecutions.
Jackson did not rule from the bench.