Robert "Joe" Halderman, who pleaded guilty to the blackmail accusation, was let go early for good behavior after serving four months of his six-month sentence, NY1 reported.
Halderman admitted to attempting to blackmail Letterman for $2 million over extramarital affairs he had with staff members, NY1 said.
Halderman's lawyer said he is happy to be out of prison and is looking for a job.
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