"He upped the offer to perform at the wedding and told Lana that it's his and Kim's special song and it would mean so much to have her perform it. She couldn't say no," a source told U.K. tabloid The Mirror.
She also couldn't turn down a reported six-figure fee for the show.
Kim's love for "Young and Beautiful" is a matter of public record. She first tweeted about it last summer, and then Kanye had it played by a brass band when he proposed at San Francisco's AT&T Park in October.
After Lana first turned him down, Ye reportedly hired an orchestra to perform the song, and ended up with arrangement he was happy with.
The Kardashians are now in Paris, where they'll take some friends and family on a private tour of the Palace of Versailles Friday night and then have brunch Saturday before flying everyone to Florence for the ceremony.