WASHINGTON — President Trump’s son-in-law and adviser Jared Kushner is selling his stake in Cadre, a New York-based real estate start-up that has sought to profit from a special tax break included in the package Mr. Trump signed into law in late 2017.

In selling his stake, Mr. Kushner also took advantage of a special federal program that would allow him to defer paying income taxes on any gain on his investment in Cadre, which he helped found in 2014.

As of last year, the venture had moved in at least four cities — including Los Angeles and Nashville — to invest in real estate projects in so-called opportunity zones, which were created by the Treasury Department in 2018 under a provision of the 2017 tax law that was intended to encourage new investment in low-income neighborhoods.

But a spokesman for Mr. Kushner said he was selling the stake in Cadre, valued on his financial disclosure form at between $25 million and $50 million, because his involvement with the firm was complicating efforts to bring in new investors — potentially including money from abroad — given Mr. Kushner’s White House work on international affairs.