A Long Island “sports dad” was brutally attacked by a street vendor over a measly $20 purchase outside Manhattan’s iconic Beacon Theatre, leaving him with a traumatic brain injury, according to police sources and his loved ones.

Sean Llewellyn, 47, had just left a concert outside the Upper West Side venue on Broadway near West 75th Street the night of Nov. 30 when he tried to make a purchase from a man selling knickknacks outside, the sources said.

Llewellyn handed the seller $20 – and was apparently owed change – but the shady hawker ran off with the bill, according to the sources.

When Llewellyn chased after him, the vendor was joined by several other people who began beating the dad up, the sources said.

Sean Llewellyn, 47, was brutally assaulted by a vendor outside the Upper West S

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