Sen. Bernie Sanders has made principled opposition to billionaire campaign donations a centerpiece of his 2020 presidential bid. So when the Vermont senator's campaign learned recently that it received a $470 contribution from Marta Thoma Hall, it immediately vowed to return every dollar.

Hall, wife of billionaire inventor David Hall and head of business development at Velodyne, was not happy with Sanders' refusal to accept her money.

"I think it's disappointing," Hall told Forbes on Monday. "I don't understand why they would do that. That's ridiculous."

Grassroots group People for Bernie tweeted in response to Hall:

After reviewing federal filings, Forbes reported that it "could not find any other billionaire or spouse of a billionaire who has donated to the Sanders' campaign" aside from Hall, who also gave $505 to Sen. Kamala Harris (D-Calif.) and $685 to Beto O'Rourke, who dropped out of the race earlier this month.

When Forbes notified the Sanders campaign of Hall's contributions, a Sanders spokesperson responded: "Thanks. We are returning Ms. Hall's contributions."

Prior to Hall's $470 check, Sanders stood alone in the 2020 Democratic field as the only major candidate who had not received any campaign contributions from billionaires.

"I don't think that billionaires should exist," Sanders told the New York Times in September. "Enough is enough. We are going to take on the billionaire class, substantially reduce wealth inequality in America, and stop our democracy from turning into a corrupt oligarchy."