Ian Coulter, formerly of Tughans, has been questioned under caution as part of the criminal probe into the deal.
He was interviewed on Wednesday and released without charge.
Mr Coulter left Tughans after a dispute over a success fee linked to the deal.
Nama, a state-owned so-called 'bad bank', was set up in the Republic of Ireland to take control of property loans made by the country's banks before the financial crash in 2008.
It sold all of its Northern Ireland property loans to US investment firm Cerberus in April 2014 for Â£1.24bn.
In the wake of that deal, Cerberus indirectly paid a fee of Â£7.5m to Tughans.
A large portion of that money was then moved to an Isle of Man bank account under the control Mr Coulter.
The account was allegedly intended to facilitate payments to non-lawyers or deal fixers.
That caused a major dispute between Mr Coulter and the other Tughans partners.
The money was moved back from the Isle of Man and Mr Coulter left the firm earlier this year.
Independent politician Mick Wallace made allegations about fixers' fees linked to the deal in the Irish parliament in July.
The NCA, the UK's equivalent of the FBI, then began a criminal investigation into the Nama deal after allegations about payments were made in the Irish parliament in July.
The Northern Ireland Assembly and the Irish parliament also launched inquiries into the loans sale.
National Crime Agency (NCA) detectives investigating the sale of the National Asset Management Agency's (Nama) Northern Ireland loans portfolio have interviewed a former managing partner of a Belfast legal firm.