As long as he didn’t get into a brawl or some other sort of trouble Thursday night, the president of the Transport Workers Union Local 100, Roger Toussaint, will walk free Friday morning after serving just four days in jail, Correction Department officials said.

State law allows for Mr. Toussaint to be set free on the seventh day of his 10-day sentence for good behavior; because that day falls on a Sunday, the law stipulates that he be let out Friday instead, a department spokesman, Michael Saucier, said.

Among the infractions that could have kept Mr. Toussaint in jail for the full sentence are fighting that results in an injury, fighting with a weapon, smuggling contraband, and horseplay, Mr. Saucier said.

The embattled labor leader spent Thursday like the other days of his sentence: reading, passing an hour in the rooftop recreation area, and going through the motions of his strictly scheduled day in the Manhattan Detention Center, better known as “the Tombs.” Several national labor leaders, including the president of UNITE-HERE, John Wilhelm, the secretary-treasurer of the United Auto Workers, Elizabeth Bunn, and the regional director of the UAW, Phil Wheeler, visited him Thursday.

Because he was being held in the civil unit of the jail, Mr. Toussaint was given his own cell, situated among those in trouble for minor infractions. One of his jailmates was serving time for not making childcare payments, and a neglectful landlord was serving several days for not making court-ordered repairs to his building, Mr. Saucier said earlier this week.

Mr. Toussaint may regain his freedom Friday, but his union is in dire straits. With $2.5 million in fines leveled against the TWU, and the Metropolitan Transportation Authority unwilling to accept the union’s recent revote on the original contract, it is increasingly likely that a binding contract will be hammered out by a state arbitration panel. That process could take months.