The United board gave Warnock permission to secure a permanent deal and the transfer has now gone through in time for Kabba to make his Blades debut this weekend.
Kabba told the United's official website: "It feels great, this is a new start for me and hopefully I can score some goals.
"I'll do my best, work hard for the team and the fans will also see that I have some ability as well."
Just last week Palace boss Trevor Francis recalled Kabba from a successful loan spell with Grimsby, in which he scored six goals in 14 appearances.