Updated

A 17-year-old robber’s arm was severed in Brooklyn after he held up a man at gunpoint for a pair of pricey sneakers, police sources said.

Through the app Wallapop, gunman Zachary Sam and seller Philippe Pierre, 39, had arranged to meet up at 1 p.m. Friday in Canarsie, sources said.

But when Pierre showed up, asking $490 for a pair of Air Jordan 8 Retro sneakers, the teen crook pulled out a gun and demanded the kicks for free inside Pierre’s gray Honda Pilot, police sources said.

Pierre, who was charged with attempted murder, stepped on the gas pedal, but Sam, who lives nearby, jumped out of his car at the intersection of East 86th Street and Avenue M, sources said.

Instead of speeding off, Pierre turned his car around and drove after Sam, crashing into him in front of a fence. The alleged robber’s arm was ripped off when he was pinned against the fence.

“I saw a kid under a car,” said Alex Saint Fleur, a bus driver who lives across the street.

“The guy ran him over. He got out, the driver said, ‘He’s trying to rob me. He’s trying to rob me.’”

Sam was charged with robbery and possession of a weapon. He was listed in stable condition at a nearby hospital.

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