A former Dorchester resident has been sentenced to 2½ years in a house of correction for shooting and killing a 19-year-old man during a fight at an MBTA station last year, Suffolk County prosecutors said.

Nhu Nguyen, 20, pleaded guilty Wednesday to manslaughter and firearms charges stemming from the May 7, 2011, shooting death of Derek Matulina at the Savin Hill Red Line station. Prosecutors say Nguyen and Matulina were once friends and sold marijuana together, but had a falling out over the profits.

On the day of Matulina’s death, the men encountered each other outside the station by chance, Suffolk District Attorney Daniel F. Conley’soffice said in a statement. When Nguyen demanded money he felt he was owed, Matulina challenged him to a fight, Conley’s office said. The men were separated by others when Nguyen brandished a .22-caliber pistol.

On the station platform, Matulina approached Nguyen from behind and repeatedly struck him in the head with a padlock wrapped in a bandana, prosecutors said. Nguyen then shot Matulina once in the chest, killing him, Conley’s office said.

Nguyen also received two years of probation.