Downing also ventured into territory both sides had agreed to avoid: discussion of Donald Trump's presidential campaign.
During his review, he said, he discovered that numerous foreign wire transfers did not appear on general ledgers for Manafort's political consultancy and therefore, "I was not able to trace it into the tax return".
Manafort's defense team did its darndest to sully Gates as a liar, a cheat, a thief and an adulterer for eight hours inside Judge T.S. Ellis' Alexandria courtroom over the past three days.
The judge then brought the trial to a standstill late Tuesday afternoon, calling a brief recess.
A conviction of Manafort would undermine efforts by Trump and some Republican lawmakers to paint Mueller's inquiry as a political witch hunt, while an acquittal would be a setback for the special counsel.
In general, references to Trump at the trial have been few and far between. He also is accused of fraudulently obtaining millions more in bank loans, including while he was working on the Trump campaign.
Manafort's accountant was asked about a purported $10 million loan from Russian oligarch Oleg Deripaska.
Manafort defense attorney Kevin Downing tried to confront Gates over whether he had engaged in four extra-marital affairs and failed to disclose them to prosecutors before becoming a witness for the government. They have tried several times to impugn his credibility before the jury.
Ellis has also criticized prosecutors for presenting too much evidence related to Manafort's spending, pointing out it isn't a crime to spend on luxuries.
Gates' testified that he and Manafort knew they were committing crimes for years.
Welch said he arrived at the $16.5 million figure based on an accounting method used by Manafort.
Cindy Laporta testified Friday in the government's case against the former Trump campaign chairman, saying she went along with the scam because she was anxious Manafort and his right-hand-man Rick Gates would sue.
Gates admitted to hiding his own income from the IRS in offshore bank accounts.
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Gates appeared to be referring to his decision to enter into a plea agreement with the office of Special Counsel Robert Mueller in exchange for his trial testimony.
Gates said under questioning Wednesday that he told Federal Bureau of Investigation agents and Justice Department lawyers about some of the offshore companies that contained millions of dollars in proceeds from their Ukrainian political work.
Gates came on board the Trump campaign with Manafort and stayed for the duration of the campaign, later serving as deputy chairman for Trump's inaugural committee and then starting a pro-Trump PAC.
On Tuesday, Downing asked how the jury could believe Gates "after all the lies you told and fraud you committed".
For this Manafort was paid US$4million a year, in quarterly payments of US$1million, Gates recalled, though at some point the currency switched from United States dollars to Euros.
In December 2016, Manafort sent an "urgent" email to Gates including Calk and Calk's son on a list of people Manafort wanted to receive invitations to Trump's inauguration.
"I would be surprised, given what's occurred in the Manafort trial, if Judge Ellis didn't issue this relatively standard instruction", Abramson said. Courtroom illustration courtesy of Bill Hennessy.
The accountant testified she was unaware if the loan was recognized as income or ever paid back.
Lawyers for Manafort used the opportunity to scrutinize Gates' personal foibles, asking if he had a "secret life" from 2010 to 2014.
In one email, Manafort wrote "WTF" and "not happy" about tax payments he was going to have to make, Gates said.
In other testimony, Gates recounted how he converted a PDF of a profit-and-loss statement to a Microsoft Word document so he could doctor it to inflate the business' income. Gates also fabricated a forgiveness letter for what he said was already a fake loan between Manafort's consulting company and a Cypriot entity he controlled. "Mr Manafort had the same path".
For most of his testimony, Gates did not look at Mr Manafort, while the defendant stared intently at his former partner.
Gates (46) admitted he had embezzled from other employers; he had volunteered the information once he began cooperating.