Hedge fund mogul Bill Ackman is poised to make a pretty penny on one of his New York City real estate investments, The Post has learned.

Manhattan’s fall deal season is kicking off with a Madison Avenue office building near upscale Barneys going up for grabs in a sale that could fetch $120 million, sources said.

The 22-story office building at 645 Madison on the southeast corner of East 60th Street is being marketed by Eric Anton of Marcus & Millichap, sources said.

The sellers are Friedland Properties and Nightingale Properties, along with Ackman, head of Pershing Square Capital Management, sources said. They purchased the building’s ground lease in 2015 for $76 million from TF Cornerstone, the sources added.

Ackman’s stake in the building, which he owns personally and not through one of his investment companies, was not immediately clear. He did not return a request for comment.

The approximately 164,000-square-foot building is being upgraded with new windows and a new lobby on East 60th Street. This adds space for another retailer on Madison Avenue, where stunning new glass storefronts and a new facade are being installed.

Accessories and perfume retailer Byredo has already leased one of the building’s newly reimagined retail spaces. The renovation and teeming area foot traffic attracted Byredo, which has a store in Soho at 62 Wooster St. and is sold at swanky stores like Barneys and Space NK, sources said.

The European luxury retailer, founded in Stockholm by Ben Gorham, was represented in its 1,200- square-foot lease at the southernmost storefront by Cassie Durand at CBRE. The retail lease was negotiated for the ownership by the Ripco team of Gene Spiegelman, Beth Rosen and Richard Skulnik.

Asking rents are $1,350 per foot for the 17,400 feet of retail on three levels.

The offices that are being leased through CBRE are about one-third vacant with asking rents over $100 per foot. The companies declined comment.

The land is owned by Robert Baker’s NRDC and leased for another 60 years. A December 2018 arbitration set the land value at $163 million, court documents show, which raised the yearly rent paid by the building. The amount of increase could not be learned.

This would be the fourth Madison Avenue office building sold this year, but the others are roughly five, 10 and 20 blocks away, respectively.

Divco West paid $310 million for 540 Madison Ave. at East 55th Street, and Aby Rosen bought 477 Madison at East 51st Street for $258.25 million, while the Abu Dhabi Investment Authority bought the remaining 25% stake in 330 Madison at East 43rd Street that it didn’t already own, revaluing the building at roughly $900 million.