“It was thoroughly looked at, and there’s a reason there were no charges brought, because there was nothing there,” Mr. de Blasio said in January.

On Thursday, a mayoral spokesman reiterated Mr. de Blasio’s stance, saying, “This administration acted appropriately at all times, as we’ve said several thousand times.”

But the testimony on Thursday raised new questions.

Mr. Singh testified that he met Mr. de Blasio when he was the city’s public advocate. At the time, Mr. Singh said, he was having issues with his Water’s Edge restaurant, which was on city property in Long Island City, Queens.

The restaurateur was behind on the rent, and a dispute had arisen over who was responsible for the replacement of a nearby pier. Mr. Singh’s restaurant would benefit from the work, but he did not want to pay for it.

He also wanted to ensure that the lease would be renewed.

Mr. Singh said he called Thomas J. Garry, a lawyer and Democratic Party official, who knew Mr. de Blasio. The three met for lunch at Water’s Edge in the summer of 2011 or 2012, the restaurateur said.

The lunch started what would eventually become “a very close relationship,” according to Mr. Singh.

As Mr. de Blasio and his staff arranged for meetings with officials, the restaurateur said he started fund-raising at Mr. Garry’s request. Two events while Mr. de Blasio was public advocate each brought in $10,000, Mr. Singh said, and four more during the mayoral campaign added between $30,000 and $60,000, he said.

Then there were the personal campaign contributions. Records show that Mr. Singh, his family and other associates collectively contributed tens of thousands of dollars. But the restaurateur suggested Thursday he gave more through “straw donors” — people who contribute to campaigns but then are reimbursed by the actual donor.