Lamard Joye was celebrating his 35th birthday with friends last month near a basketball court in Coney Island, Brooklyn. At some point, the police arrived and stopped a friend of his.

From several feet away, Mr. Joye objected. What happened next is now under investigation by the Brooklyn district attorney’s office, which is in possession of a cellphone video of the ensuing confrontation.

The video of the encounter, on Sept. 16 at around 12:20 a.m., shows a police officer steering Mr. Joye against a chain-link fence to pat him down. “Look,” Mr. Joye says. “Look, you see this? Look.” The police officer reaches into Mr. Joye’s pocket, removes what appears to be a folded stack of bills and steps back.

“Give me my money,” Mr. Joye says in the video. The police officer then sprays something at him.

Mr. Joye said the officer took more than $1,000 in cash and deployed pepper spray.