I always thought this could have been included in the film. It is referred to in the conversation the senator has with Michael although it had not yet occurred but the senator knew of plans for Klingman to be removed.
One might have thought that initial legal offers had been made to Klingman for his ownership or his share of ownership in the casino, but the man sounds genuinely surprised by Al Neri -- it is as if this is the first he is hearing of even Coreleone's interest in buying and is genuinely outraged by Neri's behavior.
The thing that allows this to even occur is that none of the security guards do anything -- had they intervened, Klingman could have made himself scarce in the future in the casino or had bodyguards and he would never have had to sign over the casino.
His final line is, however, "Okay, I'll clear out" and this almost sounds like he either knew all along what was going on or just finally accepted that he could do nothing about it.
But how realistic is this? One could certainly not expect something like this to work at any normal company, like GM or US Steel or Microsoft -- huge amounts of paperwork associated with selling a company or even a house, probably rules that arose long ago to prevent things like this. At some point Klingman would have to sign something and not only that, if the transaction greatly undervalued the casino, he could later sue.
Presumably also Al Neri had had to bribe all the security guards -- they tend to be loyal to the man who pays their salaries.
Maybe the implausibility of the scene is why it was cut. On the other hand, it is implied that the Coreleones managed to force a buyout of Moe Greene by the expedient of murdering him and presumably his share reverted to someone who was amenable to selling.