According to HillBuzz,
It’s actually quite common for very wealthy friends of a President to buy him a new home for after he leaves the White House, since most of the people who’ve been President so far were not very rich before they entered office (with Obama being an exception since he had several millions from his book sales before his election).
The woman who is organizing the funding of the Obama’s post-presidential estate is Penny Pritzker, who is also in charge of raising the donations for Obama’s presidential library and museum. The two are more or less the same project, with the priority being to get the $35 million together to fund the purchase of the Obamas’ new home. It appears that Pritzker herself may contribute up to half of the total purchase price, making up the difference from whatever the Obamas could not afford and the fundraising wouldn’t cover. Pritzker is a billionaire who in recent years resolved a long-standing family feud over her late father’s vast business empire that includes the Hyatt Hotel chain as well as assorted manufacturing operations. The wealthiest families in Chicago are being leaned on heavily to contribute with Penny aggressively guilting them with “appeals of you don’t want the President to be homeless in January, do you?”. That’s more or less an exact quote from Penny, if my source within her social group is not paraphrasing.