The particular project in question is one dear to Trump's heart -- the 1980 demolition of a building to clear the way for his eponymous tower on Manhattan's Fifth Avenue.
During Thursday night's debate, Rubio claimed that Trump had to pay $1 million after using undocumented Polish workers, but it is unclear how Rubio came up with that number.
The lawsuit in question -- which claimed that the workers were underpaid and that money wasn't paid into a union benefits fund -- was first filed in 1983 and the trial lasted for 16 days in the summer of 1990, according to court records.
During the trial, The New York Times reported that Trump testified he didn't know that the 200 workers who worked for the contractor hired to do the demolition of the Bonwit Teller building so construction could start on the Trump Tower, were undocumented immigrants.
Instead, he said that the workers were hired by Kaszycki & Sons Contractors.
In the middle of the 15-year civil case, the attorney for the plaintiffs said they were seeking money to be put into benefit funds for union pensions and were expecting a $1 million to $2 million payment. The case was reportedly settled in 1999, but there was no public disclosure about any settlement or amount.
Trump's team did not respond to ABC News' requests for comment.
When he responded during the debate, Trump did not disclose the terms of any purported settlement.
"I've hired tens of thousands of people. He brings up something from 30 years ago, it worked out very well. Everybody was happy," Trump said.