think your needing to go do some research, United were not willing to pay the then british transfer record his agent wanted for him, he only went to city as they paid the highest fee, simply he was a better player then Berbatov and suited our style at the time alot more.
His agent/owner wanted too much money. United were not willing to play ball. But since he had a lot of control over Tevez, he was able to push very hard. Once it didn't work out, City were happy to put salt in the wound.
Apart from that,
He scored 5 league goals in 29 games in his second season at United. A total of 15 goals in 51 games that season, not a particularly great return given his form the previous season.
Ferguson was also very misty-eyed over Berbatov, evidently felt with Tevez's previous season form he needed to shake things up a bit. I think Ferguson saw something of Cantona in Berbatov and was massively determined to sign him.
There's also evident from Gary Neville that he downed tools during the 2nd season.
EDIT:
Add to all of that, Tevez (not appreciating the club's mystique), ridiculous transfer fees, wages, Fergie prepared for the inevitable and signed Berba before something bad would happen.
Who could blame Fergie? Berba was one of the elite forward in the PL during that time, and he was available, might as well take the punt and top up the squad.
In two seasons, he lost Tevez, and then Ronaldo, Berba was the top forward available at the time. People tend to forget that important development why we eventually signed Berba.