John Manchin, a physician, says the senator and their brother, Roch, borrowed $1.7 million from him in the 1980s when all three were equal partners in the Manchin Brothers business.
Joe and Roch Manchin also owned a carpet businesses that struggled in the late 80s, when John Manchin leant them the money to keep the business afloat, the lawsuit says.
"In spite of their numerous and repeated renewed promises to and agreements with Plaintiff over time to do so, Defendants Joseph Manchin III and Roch Manchin negligently, intentionally, and willfully failed and refused to repay Plaintiff," the suit reads.
The lawsuit points to a 2012 funds-sharing agreement in which the West Virginia Democrat promises to pay to John Manchin the first $1 million in profit of a power plant project in which the senator had invested. The power plant project has not moved forward.
John Manchin alleges the agreement was meant to purposely deceive him and that it was reorganized with all funds and assets transferred to the control of the senator and his son, Joseph Manchin IV.
A spokesman for the senator's office called the issue a "family matter" and declined to comment further.