It's not hard to understand how Paul Manafort could work for the Trump campaign as a "volunteer". Via Reuters:
A search warrant application unsealed on Wednesday revealed closer links than previously known between President Donald Trump’s former campaign manager Paul Manafort and a Russian oligarch with close ties to the Kremlin.
In an affidavit attached to the July 2017 application, an FBI agent said he had reviewed tax returns for a company controlled by Manafort and his wife that showed a $10 million loan from a Russian lender identified as Oleg Deripaska.
The application to search Manafort’s Virginia apartment was granted, providing key evidence for Special Counsel Robert Mueller’s indictments of Manafort as part of his investigation into Russia’s alleged meddling in the 2016 presidential campaign.