The 21 year old traveled to Sheffield on Friday to discuss personal terms after Neil Warnock agreed a fee with Eagles chairman Michael Jordon.
United tried to get Kabba on a loan deal on Wednesday but this deal fell through as the player was looking for a permanent deal.
After a very successful loan spell at Grimsby Town the Selhurst Park called back the striker who had scored six in fourteen games for the Mariners due lack of strikers.
However ex-piggy boss Trevor Francis decided to let Kabba move on despite claiming that he is "Palace through and through".
The Blades managed to sign him on time so he could start against Reading tomorrow through I would expect that he will start on the bench.