What dream does Mr. Liner mention? Act II, scene 3.
Linder and the people he represents "dream of the kind of community they want to raise their children in." They are not wealthy, but probably working-class folk who see Clybourne Park as their own first step up the ladder of middle-class success.
It is very important not to demonize Mr. Linder. Hansbury has taken great pains to present him as shy, reasonable, and in all respects (save one) respectable. He doesn't want to hurt the Youngers, and believes truly that he is doing the right thing by offering them an option. As he phrases it, the dream of having a safe place to raise one's children is identical to the dream expressed by Mama. That is the point.
He does not say or imply that he will not be responsible if certain elements "get worked up." It is a misreading to suggest that he is making threats. His last line is not a warning, but a simple "I hope you folks know what you're doing."
Doubtless Hansbury wanted to avoid alienating white investors and audience, and so deliberately worked to make Linder a non-threatening character. But by giving him the language she does, and constructing Linder's arguments as she does, she shows that the Youngers and their white neighbors-to-be actually have exactly the same dream.
Karl Lindner is the racist representative of the Clybourne Park Improvement Association. He dreams of people living together "harmoniously". Of course, living "harmoniously" will only happen if the black people stay in their own neighborhoods and not try to move into the white ones. He refers to the Youngers as "you people" while trying to convince them not to buy the house. He even offers them money to buy their house. Then he threatens them by saying he won't be responsible if some of the neighbors get all "worked up" and do something violent. This is a serious event in the play because many whites were furious that blacks wanted to move into their neighborhoods. Violence did occur in several states across the U.S.