Last month, real estate blogs, Philadelphia magazine and other publications reported on a rumor that music’s power couple Beyoncé and Jay Z had purchased a $17.6 million penthouse at 500 Walnut, a condominium project under construction in Philadelphia.

Tom Scannapieco, the project’s developer, said he doesn’t know where the rumor—which he said is untrue—came from. “We never even had a conversation” with the stars, he said. But “if I can find out, I’m going to hire the person,” he added with a laugh—as the tale generated...