Nhu Nguyen, 20, pleaded guilty Wednesday to killing Derek Matulina during a fight at the Savin Hill Red Line stop on May 11, 2011.

Under an agreement between his attorney and the Suffolk County District Attorney's office, Nguyen was sentenced to 2 1/2 years in a county jail on manslaughter and gun charges, to be followed by two years of probation, at his sentencing in Suffolk Superior Court.

The DA's office says the two were once friends who had a falling out over the proceeds of marijuana sales.

The two meet up by chance at Savin Hill, where Nguyen demanded what he felt he was owed, the DA's office reports. Matulina challenged him to a fight, at which point Nguyen pulled out a .22 caliber handgun. Acquaintances separated the two, but then Matulina followed him onto the subway platform, striking him repeatedly with a padlock wrapped in a bandana, prosecutors say.

Matulina then grabbed Nguyen, who managed to escape, pull out his gun and shoot Matulina in the chest, the DA's office says.